discovery in 1847 of the process of partial yolk segmentation in | researches (see below) that the germinal layer theory was the germinal disk of the fowl's egg during its descent through the oviduct, and his observations on the same phenomenon in fishes and mammals. The development of reptiles received important elucidation from the researches of Rathke, in his history of the development of serpents, published at Königsberg in 1839, and in a similar work on the turtle in 1848, as well as in a later one on the crocodile in 1866, along with which may be associated the observations of H. J. Clark on the "Embryology of the Turtle," published in Agassiz's Contributions to Natural History, &c., 1857. The phenomena of yolk segmentation, to which reference has more than once been made, and to which later researches give more and more importance in connexion with the fundamental phenomena of development, received great elucidation during this period, first from the observations of C. T. E. von Siebold and those of Bagge on the complete yolk segmentation of the egg in nematoid worms in 1841, and more fully by the observations of Kölliker in the same animals in 1843. The nature of partial segmentation of the yolk was first made known by Kölliker in his work on the development of the Cephalopoda in 1844, and, as has already been mentioned, the phenomena were observed by Coste in the eggs of birds. The latter observations have since been confirmed by those of Oellacher, Götte and Kölliker. Further researches in a vast number of animals give every reason to believe that the phenomenon of segmentation is in some shape or other the invariable precursor of embryonic formation. applied to the embryos of all the Metazoa. But the year 1859 will be for ever memorable in the history of science as the year of the publication of the Origin of Species. If the enunciation of the cell-theory may be said to have marked a first from a second period in the history of embryology, the publication of Darwin's great idea ushered in a third. Whereas hitherto the facts of anatomy and development were loosely held together by the theory of types which owed its origin and maintenance to Cuvier, L. Agassiz, J. Müller and R. Owen, they were now combined into one organic whole by the theory of descent and by the hypothesis of recapitulation which was deduced from that theory. First clearly enunciated by Johann Müller in his well-known work Für Darwin published in 1864 (rendered in England as Facts for Darwin, 1869), the view that a knowledge of embryonic and larval histories would lay bare the secrets of race history and enable the course of evolution to be traced and so lead to the discovery of the natural system of classification, gave a powerful stimulus to embryological research. The first fruits of this impetus were gathered by Alexander Agassiz, A. Kowalewsky and E. Metschnikoff. Agassiz, in his memoir on the Embryology of the Starfish published in 1864, showed that the body-cavity in Echinodermata arises as a differentiation of the enteron of the larva and so laid the foundations of our present knowledge of the coelom. This discovery was confirmed in 1869 by Metschnikoff (“Studien üb. d. Entwick. d. Echinodermen u. Nemertinen," Mém. Ac. Pétersbourg (7), 41, 1869), and extended by him to Tornaria, the larva of Balanoglossus in 1870 ("Untersuchungen üb. d. Metamorphose einiger Seethiere," Zeit. f. wiss. Zoologie, 20, 1870). In 1871 Kowalewsky in his classical memoir, entitled " Embryologische Studien an Würmern und Arthropoden (Mém. Acad. Pétersbourg (7), 16, 1871), proved the same fact for Sagitta and added immensely to our knowledge of the early stages of development of the Invertebrata. These memoirs formed the basis on which subsequent workers took their stand. Amongst the most important of these was F. M. Balfour (1851-1882). Led to the study of embryology by his teacher, M. Foster, in association with whom he published in 1874 the Elements of Embryology, Balfour was one of the first to take advantage of the facilities for research offered by Dr. A. Dohrn's Zoological Station at Naples which has since become so celebrated. Here he did the work which was subse The first considerable work on the development of a division of the invertebrates was that of Maurice Herold of Marburg on spiders, De generatione aranearum ex ovo, published at Marburg in 1824, in which the whole phenomena of the formative processes in that animal are described with remarkable clearness and completeness. A few years later an important series of contributions to the history of the development of invertebrate animals appeared in the second volume of Burdach's work on Physiology, of which the first edition was published in 1828, and in this the history of the development of the Entozoa was the production of Ch. Theod. von Siebold, and that of most of the other invertebrates was compiled by H. Rathke from the results of his own observations and those of others. These memoirs, together with others subsequently published by Rathke, notably that Über die Bildung und Entwickelungs-quently published in 1878 in his Monograph of the Development geschichte d. Flusskrebses (Leipzig, 1829), in which an attempt is made to extend the doctrine of the derivation of the organs from the germinal layers to the invertebrata, entitle him to be regarded as the founder of invertebrate embryology. A large body of facts having by this time been ascertained with respect to the more obvious processes of development, a further attempt to refer the phenomena of organogenesis to morphological and histological principles became desirable. More especially was the need felt to point out with greater minuteness and accuracy the relation in which the origin of the fundamental organs of the embryo stands to the layers of the blastoderm; and this we find accomplished with signal success in the researches of R. Remak on the development of the chick and frog, published between the years 1850 and 1855. Starting from Pander's discovery of the trilaminate blastoderm, Remak worked out the development of the chick in the light of the cell-theory of Schleiden and Schwann. He observed the division of the middle layer into two by a split which subsequently gives rise to the body-cavity (pleuro-peritoneal space) of the adult; and traced the principal organs which came from these two layers (Hautfaserblatt and Darmfaserblatt) respectively. In this manner the foundations of the germ-layer theory were established in their modern form. A great step forward was made in 1859 by T. H. Huxley, who compared the serous and mucous layers of Pander with the ectoderm and endoderm of the Coelenterata. But in spite of this comparison it was generally held that germinal layers similar to those of the vertebrata were not found in invertebrate animals, and it was not until the publication in 1871 of Kowalewsky's of Elasmobranch Fishes, and which constituted the most important addition to vertebrate morphology since the days of Johannes Müller. This was followed in 1879 and 1881 by the publication of his Treatise on Comparative Embryology, the first work in which the facts of the rapidly growing science were clearly and philosophically put together, and the greatest. The influence of Balfour's work on embryology was immense and is still felt. He was an active worker in every department of it, and there are few groups of the animal kingdom on which he has not left the impress of his genius. In the period under consideration the output of embryological work has been enormous. No group of the animal kingdom has escaped exhaustive examination, and no effort has been spared to obtain the embryos of isolated and out of the way forms, the development of which might have a bearing upon important questions of phylogeny and classification. Of this work it is impossible to speak in detail in this summary. It is only possible to call attention to some of its more important features, to mention the more important advances, and to refer to some of the more striking memoirs. Marine zoological stations have been established, expeditions have been sent to distant countries, and the methods of investigation have been greatly improved. Since Anton Dohrn founded the Stazione Zoologica at Naples in 1872, observatories for the study of marine organisms have been established in most countries. Of journeys' which have been made to distant countries and which have resulted in important contributions to embryology, may be mentioned the expedition (1884-1886) of the cousins Sarasin to Ceylon (development of Gymnophiona), discovery in 1880 (Quart. Journ. Mic. Sci., 20, p. 164) that the nephridia of Elasmobranchs are a direct differentiation of a portion of it. In 1886 this was extended to Peripatus (Proc. Camb. Phil. Soc., 6, p. 27) and doubtless holds universally. of E. Selenka to Brazil and the East Indies (development of | creased precision was given to the conception of coelom by the Marsupials, Primates and other mammals, 1877, 1889, 1892), of A. A. W. Hubrecht to the East Indies (1890, development of Tarsius), of W. H. Caldwell to Australia (1883-1884, discovery of the nature of the ovum and oviposition of Echidna and of Ceratodus), of A. Sedgwick to the Cape (1883, development of Peripatus), of J. Graham Kerr to Paraguay (1896, development of Lepidosiren), of R. Semon to Australia and the Malay Archipelago (1891-1893, development of Monotremata, Marsupialia), and of J. S. Budgett to Africa (1898, 1900, 1901, 1903, development of Polypterus). In methods, while great improvements have been made in the processes of hardening and staining embryos, the principal advance has been the introduction in 1883 by W. H. Caldwell in his work on the development of Phoronis of the method of making tape-worm like strings of sections as a result of which the process of mounting in order all the sections obtained from an embryo was much facilitated, and the use of an automatic microtome rendered possible. The method of Golgi for the investigation of the nervous system, introduced in 1875, must | also be mentioned here. 66 The word coelom" (q.v.) was introduced into zoology by E. Haeckel in 1872 (Kalkschwämme, p. 468) as a convenient term for the body-cavity (pleuroperitoneal). The word was generally adopted, and was applied alike to the blood-containing body-cavity of Arthropods and to the body-cavity of Vertebrata and segmented worms, in which there is no blood. In 1875 Huxley (Quarterly Journ. of Mic. Science, 15, p. 53), relying on the researches of Agassiz, Metschnikoff and Kowalewsky above mentioned, put forward the idea that according to their development three kinds of body-cavity ought to be distinguished: (1) the enterocoelic which arises from enteric diverticula, (2) the schizocoelic which develops as a split in the embryonic mesoblast, and (3) the epicoelic which was enclosed by folds of the skin and lined by ectoderm (e.g. atrial cavity of Tunicates, &c.). This suggestion was of great importance, because it led the embryologists of the day (Balfour, the brothers Hertwig, Lankester and others) to discuss the question as to whether there was not more than one kind of body-cavity. The Hertwigs (Coelomtheorie, Jena, 1881) distinguished two kinds, the enterocoel and the pseudocoel. The former, to which they limited the use of the word coelom, and which is developed directly or indirectly from the enteron, is found in Annelida, Arthropoda, Echinodermata, Chordata, &c. The latter they regarded as something quite different from the coelom and as arising by a split in what they called for the first time mesenchyme; the mesenchyme being the non-epithelial mesoderm, which they described as consisting of amoeboid cells, but which we now know to consist of a continuous reticulum. The next step was made by E. Ray Lankester, who in 1884 (Zoologischer Anzeiger) showed that the pericardium of Mollusca does not contain blood, and therein differs from the rest of the body-cavity which does contain blood, but no suggestion is made that the blood-containing space is not coelomic. In fact it was generally held by the anatomists of the day that the coelom and the vascular system were different parts of the same primitive organ, though separate from it in the adult except in Arthropoda and Mollusca. In the Mollusca, it is true, the pericardial part of the coelom was held to be separate from the vascular, and the Hertwigs had reached the correct conception that the pericardium of these animals was alone true coelom, the vascular part being pseudocoel. This was the state of morphological opinion until 1886, when it was shown (Proc. Cambridge Phil. Soc., 6, 1886, p. 27) (1) that the coelom of Peripatus gives rise to the nephridia and generative glands only, and to no other part of the bodycavity of the adult, (2) that the nephridia of the adult do not open as had been supposed into the body-cavity, (3) that the bodycavity is entirely formed of the blood-containing space, the coelom having no perivisceral portion. These results were extended by the same author (Quart. Journ. Mic. Sci., 27, 1887, pp. 486-540) to other Arthropods and to the Mollusca, and the modern theory of the coelom was finally established. An in In 1864 it was suggested by V. Hensen (Virchow's Archiv, 31) that the rudiments of nerve-fibres are present from the beginning of development as persistent remains of connexions between the incompletely separated cells of the segmented ovum. This suggestion fell to the ground because it was held by embryologists that the cleavage of the ovum resulted in the formation of completely separate cells, and that the connexions between the adult cells were secondary. In 1886 it was shown (Quarterly Journ. Mic. Sci., 26, p. 182) that in Peripatus Capensis the cells of the segmenting ovum do not separate from one another, but remain connected by a loose protoplasmic network. This discovery has since been extended to other ova, even to the small socalled holoblastic ova, and a basis of fact was found for Hensen's suggestion as to the embryonic origin of nerves (Quart. Journ. Mic. Sci., 33, 1892, pp. 581-584). An extension and further application of the new views as to the cell-theory and the embryonic origin of nerves thus necessitated was made in 1894 (Quart. Journ. Mic. Sci., 37, p. 87), and in 1904 J. Graham Kerr showed that the motor nerves in the dipnoan fish Lepidosiren arise in an essentially similar manner (Trans. Roy. Society of Edinburgh, 41, p. 119). In 1883 Elie Metschnikoff published his researches on the intracellular digestion of invertebrates (Arbeiten a. d. zoologischen Inst. Wien, 5; and Biologisches Centralblatt, 3, p. 560); these formed the basis of his theory of inflammation and phagocytosis, which has had such an important influence on pathology. As he himself has told us, he was led to make these investigations by his precedent researches on the development of sponges and other invertebrates. To quote his own words: "Having long studied the problem of the germinal layers in the animal series, I sought to give some idea of their origin and significance. The part played by the ectoderm and endoderm appeared quite clear, and the former might reasonably be regarded as the cutaneous investment of primitive multicellular animals, while the latter might be regarded as their organ of digestion. The discovery of intracellular digestion in many of the lower animals led me to regard this phenomenon as characteristic of those ancestral animals from which might be derived all the known types of the animal kingdom (excepting, of course, the Protozoa). The origin and part played by the mesoderm appeared the most obscure. Thus certain embryologists supposed that this layer corresponded to the reproductive organs of primitive animals: others regarded it as the prototype of the organs of locomotion. My embryological and physiological studies on sponges led me to the conclusion that the mesoderm must function in the hypothetically primitive animals as a mass of digestive cells, in all points similar to those of the endoderm. This hypothesis necessarily attracted my attention to the power of seizing foreign corpuscles possessed by the mesodermic cells" (Immunity in Infective Diseases, English translation, Cambridge, 1905). The branch of embryology which concerns itself with the study of the origin, history and conjugation of the individuals (gametes) which are concerned in the reproduction of the species has made great advances. These began in 1875 and following years with a careful examination of the behaviour of the germinal vesicle in the maturation and fertilization of the ovum. The history of the polar bodies, the origin of the female pronucleus, the presence in the ovum of a second nucleus, the male pronucleus, which gave rise to the first segmentation nucleus by fusion with the female pronucleus, were discovered (E. van Beneden, O. Bütschli, O. Hertwig, H. Fol), and in 1876 O. Hertwig (Morphologisches Jahrbuch, 3, 1876) for the first time observed the entrance of a spermatozoon into the egg and the formation of the male pronucleus from it. The centrosome was discovered by W. Flemming in 1875 in the egg of the fresh-water mussel, and independently in 1876 by E. van Beneden in Dicyemids. In 1883 came E. van Beneden's celebrated discovery (Arch. Biologie, 4) of the reduction of the number of chromosomes in the nucleus of both male and female gametes, and of the fact that the male and female pronuclei contribute the same number of chromosomes to the zygote-nucleus. He also showed that the gametogenesis in the male is a similar process to that in the female, and paved the way for the acceptation of the view (due to Bütschli) that polar bodies are aborted female gametes. These discoveries were extended and completed by subsequent workers, among whom may be mentioned E. van Beneden, J. B. Carnoy, G. Platner, T. Boveri, O. Hertwig, A. Brauer. The subject is still being actively pursued, and hopes are entertained that some relation may be found between the behaviour of the chromosomes and the facts of heredity. Since 1874 (W. His, Unsere Körperform und das physiologische Problem ihrer Entstehung) a new branch of embryology, which concerns itself with the physiology of development, has arisen (experimental embryology). The principal workers in this field have been W. Roux, who in 1894 founded the Archiv für Entwickelungsmechanik der Organismen, T. Boveri and Y. Delage who discovered and elucidated the phenomenon of merogony, J. Loeb who discovered artificial parthenogenesis, O. and R. Hertwig, H. Driesch, C. Herbst, E. Maupas, A. Weismann, T. H. Morgan, C. B. Davenport (Experimental Morphology, 2 vols., 1899) and many others. In the elucidation of remarkable life-histories we may point in the first place to the work of A. Kowalewsky on the development of the Tunicata ("Entwickelungsgeschichte d. einfachen Ascidien," Mém. Acad. Pétersbourg (7), 10, 1866, and Arch. f. Mic. Anatomie, 7, 1871), in which was demonstrated for the first time the vertebrate relationship of the Tunicata (possession of a notochord, method of development of the central nervous system) and which led to the establishment of the group Chordata. We may also mention the work of Y. Delage in the metamorphosis of Sacculina (Arch. zool. exp. (2) 2, 1884), A. Giard (Comptes rendus, 123, 1896, p. 836) and of A. Malaquin on Monstrilla (Arch. zool. exp. (3), 9, p. 81, 1901), of Delage (Comptes rendus, 103, 1886, p. 698) and Grassi and Calandruccio (Rend. Acc. Lincei (5), 6, 1897, p. 43), on the development of the eels, and of P. Pergande on the life-history of the Aphidae (Bull. U.S. Dep. Agric. Ent., technical series, 9, 1901). The work of C. Grobben (Arbeiten zool. Inst. Wien, 4, 1882) and of B. Uljanin ("Die Arten der Gattung Doliolum," Fauna u. Flora des Golfes von Neapel, 1884) on the extraordinary life-history and migration of the buds in Doliolum must also be mentioned. In pure embryological morphology we have had Heymons' elucidation of the Arthropod head, the work of Hatschek on Annelid and other larvae, the works of H. Bury and of E. W. MacBride which have marked a distinct advance in our knowledge of the development of Echinodermata, of K. Mitsukuri, who has founded since 1882 an important school of embryology in Japan, on the early development of Chelonia and Aves, of A. Brauer and G. C. Price on the development of vertebrate excretory organs, of Th. W. Bischoff, E. van Beneden, E. Selenka, A. A. W. | Hubrecht, R. Bonnet, F. Keibel and R. Assheton on the development of mammals, of A. A. W. Hubrecht and E. Selenka on the early development and placentation of the Primates, of J. Graham Kerr and of J. S. Budgett on the development of Dipnoan and Ganoid fishes, of A. Kowalewsky, B. Hatschek, A. Willey and E. W. MacBride on the development of Amphioxus, of B. Dean on the development of Bdellostoma, of A. Götte on the development of Amphibia, of H. Strahl and L. Will on the early development of reptiles, of T. H. Huxley, C. Gegenbaur and W. K. Parker on the development of the vertebrate skeleton, of van Wijhe on the segmentation of the vertebrate head, by which the modern theory of head-segmentation, previously adumbrated by Balfour, was first established, of Leche and Röse on the development of mammalian dentitions. We may also specially notice W. Bateson's work on the development of Balanoglossus and his inclusion of this genus among the Chordata (1884), the discovery by J. P. Hill of a placenta in the marsupial genus Perameles (1895), the work of P. Marchal (1904) on the asexual increase by fission of the early embryos of certain | parasitic Hymenoptera (so called germinogony), a phenomenon which had been long ago shown to occur in Lumbricus trapezoides by N. Kleinenberg (1879) and by S. F. Harmer in Polyzoa (1893). The work on cell-lineage which has been so actively pursued in America may be mentioned here. It has consisted mainly of an extension of the early work of A. Kowalewsky and B. Hatschek on the formation of the layers, being a more minute and detailed examination of the origin of the embryonic tissues. The most important text-books and summaries which have appeared in this period have been Korschelt and Heider's Lehrbuch der vergleichenden Entwickelungsgeschichte der wirbellosen Tiere (1890-1902), C. S. Minot's Human Embryology (1892), and the Handbuch der vergleichenden und experimentellen Entwickelungslehre der Wirbeltiere, edited by O. Hertwig (1901, et seq.). See also K. E. von Baer, Über Entwicklungsgeschichte der Tiere (Königsberg, 1828, 1837); F. M. Balfour, A Monograph on the Development of Embryology, vols. i. and ii. (London, 1885) (still the most important Elasmobranch Fishes (London, 1878); A Treatise on Comparative work on Vertebrate Embryology); M. Duval, Atlas d'Embryologie (Paris, 1889); M. Foster and F. M. Balfour, Elements of Embryology (London, 1883); O. Hertwig, Lehrbuch der Entwicklungsgeschichte des Menschen u. der Wirbeltiere (6th ed., Jena, 1898); A. Kölliker, 1879); A. M. Marshall, Vertebrate Embryology (London, 1893). Entwicklungsgeschichte des Menschen u. der höheren Tiere (Leipzig, (A. SE. *) PHYSIOLOGY OF DEVELOPMENT Physiology of Development [in German, Entwicklungsmechanik (W. Roux), Entwicklungsphysiologie (H. Driesch), physiologische Morphologie (J. Loeb)] is, in the broadest meaning of the word, the experimental science of morphogenesis, i.e. of the laws that govern morphological differentiation. In this sense it embraces the study of regeneration and variation, and would, as a whole, best be called rational morphology. Here we shall treat of the Physiology of Development in a narrower sense, as the study of the laws that govern the development of the adult organism from the egg, REGENERATION and VARIATION AND SELECTION forming the subjects of special articles. After the work done by W. His, A. Goette and E. F. W. Pflüger, who gave a sort of general outline and orientation of the subject, the first to study developmental problems properly in a systematical way, and with full conviction of their great importance, was Wilhelm Roux. This observer, having found by a full analysis of the facts of "development " that the first special problem to be worked out was the question when and where the first differentiation appeared, got as his main result that, when one of the two first blastomeres (cleavage cells) of the frog's egg was killed, the living one developed into a typical halfembryo, i.e. an embryo that was either the right or the left part of a whole one. From that Roux concluded that the first cleavage plane determined already the median plane of the adult; and that the basis of all differentiation was given by an unequal division of the nuclear substances during karyokinesis, a result that was also attained on a purely theoretical basis by A. Weismann. Hans Driesch repeated Roux's fundamental experiment with a different method on the sea-urchin's egg, with a result that was absolutely contrary to that of Roux: the isolated blastomere cleaved like half the egg, but it resulted in a whole blastula and a whole embryo, which differed from a normal one only in its small size. Driesch's result was obtained in somewhat the same manner by E. B. Wilson with the egg of Amphioxus, by Zoja with the egg of Medusae, &c. It thus became very probable that an inequality of nuclear division could not be the basis of differentiation. The following experiments were still more fatal to the theories of Roux and of Weismann. Driesch found that even when the first eight or sixteen cells of the cleaving egg of the sea-urchin were brought into. quite abnormal positions with regard to one another, still a quite normal embryo was developed; Driesch and T. H. Morgan discovered jointly that in the Ctenophore egg one isolated blastomere developed into a half-embryo, but that the same was the case if a portion of protoplasm was cut off from the fertilized egg not yet in cleavage; last, but not of least importance, in the case of the frog's egg which had been Roux's actual subject of experiment, conditions were discovered by O. Schultze and O. Hertwig under which one of the two first blastomeres of this egg developed | egg of Echinids. This investigator has shown that all salts into a whole embryo of half size. This result was made still of the sea water are of great importance for development, and more decisive by Morgan, who showed that it was quite in the most of them specifically and typically; for instance, calcium power of the experimenter to get either a half-embryo or a whole is absolutely necessary for holding together the embryonic cells, one of half size, the latter dependent only upon giving to the and without calcium all cells will fall apart, though they do not blastomere the opportunity for a rearrangement of its matter die, but live to develop further. by turning it over. Thus we may say that the general result of the introductory series of experiments in the physiology of development is the following:-In many forms, e.g. Echinoderms, Amphioxus, Ascidians, Fishes and Medusae, the potentiality (prospective Potenz-Driesch) of all the blastomeres of the segmented egg is the same, i.e. each of them may play any or every part in the future development; the prospective value (prosp. Bedeutung D.) of each blastomere depends upon, or is a function of, its position in the whole of the segmented egg; we can term the "whole " of the egg after cleavage an aequipotential system" (Driesch). But though aequipotential, the whole of the segmented egg is nevertheless not devoid of orientation or direction; the general law of causality compels us to assume a general orientation of the smallest parts of the egg, even in cases where we are not able to see it. It has been experimentally proved that external stimuli (light, heat, pressure, &c.) are not responsible for the first differentiation of organs in the embryo; thus, should the segmented egg be absolutely equal in itself, it would be incomprehensible that the first organs should be formed at one special point of it and not at another. Besides this general argument, we see a sort of orientation in the typical forms of the polar or bilateral cleavage stages. Differentiation, therefore, depends on a primary, i.e. innate, orientation of the egg's plasma in those forms, the segmented eggs of which represent aequipotential systems; this orientation is capable of a sort of regulation or restoration after disturbances of any sort; in the egg of the Ctenophora such a regulation is not possible, and in the frog's egg it is facultative, i.e. possible under certain conditions, but impossible under others. Should this interpretation be right, the difference between the eggs of different animals would not be so great as it seemed at first: differences with regard to the potentialities of the blastomeres would only be differences with regard to the capability of regulation or restoration of the egg's protoplasm. The foundation of physiological embryology being laid, we now can shortly deal with the whole series of special problems offered to us by a general analysis of that science, but at present worked out only to a very small extent. We may ask the following questions:-What are the general conditions of development? On what general factors does it depend? How do the different organs of the partly developed embryo stand with regard to their future fate? What are the stimuli (Reize) effecting differentiation? What is to be said about the specific character of the different formative effects? And as the most important question of all: Are all the problems offered to us in the physiology of development to be solved with the aid of the laws known hitherto in science, or do we want specifically new "vitalistic" factors? of differ entiation. Energy in different forms is required for development, and is provided by the surrounding medium. Light, though of no influence on the cleavage (Driesch), has a great effect Conditions on later stages of development, and is also necessary for the formation of polyps in Eudendrium (J. Loeb). That a certain temperature is necessary for ontogeny has long been known; this was carefully studied by O. Hertwig, as was also the influence of heat on the rate of development. Oxygen is also wanted, either from a certain stage of development or from the very beginning of it, though very nearly related forms differ in this respect (Loeb). The great influence of osmotic pressure on growth was studied by J. Loeb, C. Herbst and C. H. Davenport. In all these cases energy may be necessary for development in general, or a specific form of energy may be necessary for the formation of a specific organ; it is clear that, especially in the latter case, energy is shown to be a proper factor for morphogenesis. Besides energy, a certain chemical condition of the medium, whether offered by the water in which the egg lives or (especially in later stages) by the food, is of great importance for normal ontogeny; the only careful study in this respect was carried out by Herbst for the development of the What we have dealt with may be called external factors of development; as to their complement, the internal factors, it is clear that every elementary factor of general physiology may be regarded as one of them. Chemical metamorphosis plays, of course, a great part in differentiation, especially in the form of secretions; but very little has been carefully studied in this respect. Movement of living matter, whether of cells or of intracellular substance, is another important factor (O. Bütschli, F. Dreyer, L. Rhumbler.) Cell-division is another, its differences in direction, rate and quantity being of great importance for differentiation. We know very little about it; a so-called law of O. Hertwig, that a cell would divide at right angles to its longest diameter, though experimentally stated in some cases, does not hold for all, and the only thing we can say is, that the unknown primary organization of the egg is here responsible. (Compare the papers on "cell-lineage " of E. B. Wilson, F. R. Lillie, H. S. Jennings, O. Zurstrassen and others.) Of the inner factors of ontogeny there is another category that may be called physical, that already spoken of being physiological. The most important of these is the capillarity of the cell surfaces. Berthold was the first to call attention to its role in the arrangement of cell composites, and afterwards the matter was more carefully studied by Dreyer, Driesch, and especially W. Roux, with the result that the arrangement of cells follows the principle of surfaces minimae areae (Plateau) as much as is reconcilable with the conditions of the system. Poten cells. It has already been shown that in many cases the embryo after cleavage, i.e. the blastula, is an "aequipotential system.” It was shown that in the egg of Echinids there existed such an absolute lack of determination of the cleavage tialities of cells that (a) the cells may be put in quite abnormal embryonic positions with reference to one another without disturbing development; (b) a quarter blastomere gives a quite normal little pluteus, even a sixteenth yields a gastrula; (c) two eggs may fuse in the early blastula stage, giving one single normal embryo of double size. Our next question concerns the distribution of potentiality, when the embryo is developed further than the blastula stage. In this case it has been shown that the potentialities of the different embryonic organs are different: that, for instance, in Echinoderms or Amphibians the ectoderm, when isolated, is not able to form endoderm, and so on (Driesch, D. Barfurth); but it has been shown at the same time that the ectoderm in itself, the intestine in itself of Echinoderms (Driesch), the medullary plate in itself of Triton (H. Spemann), is as aequipotential as was the blastula: that any part whatever of these organs may be taken away without disturbing the development of the rest into a normal and proportional embryonic part, except for its smaller size. If the single phases of differentiation are to be regarded as effects, we must ask for the causes, or stimuli, of these effects. For a full account of the subject we refer to Herbst, by whom also the whole botanical literature, much more important than the zoological, is critically reviewed. We have already seen that when the blastula represents an aequipotential system, there must be some sort of primary organization of the egg, recoverable after disturbances, that directs and localizes the formation of the first embryonic organs; we do not know much about this organization. Directive stimuli (Richtungsreize) play a great role in ontogeny; Herbst has analysed many cases where their existence is probable. They have been experimentally proved in two cases. The chromatic cells of the yolk sac of Fundulus are attracted by the oxygen of the arteriae (Loeb); the mesenchyme cells of Echinus are attracted by some specific parts of the ectoderm, for they move towards them also when removed from their original positions to any point of the blastocoel by shaking (Driesch). Many directive stimuli might Formative stimuli. be discovered by a careful study of grafting experiments, such as have been made by Born, Joest, Harrison and others, but at present these experiments have not been carried out far enough to get exact results. ments upon isolated blastomeres, &c., and on an analysis of the Formative stimuli in a narrower meaning of the word, i.e. stimuli affecting the origin of embryonic organs, have long been known in botany; in zoology we know (especially from Loeb) a good deal about the influence of light, gravitation, contact, &c., on the formation of organs in hydroids, but these forms are very plant-like in many respects; as to free-living animals, Herbst proved that the formation of the arms of the pluteus larva depends on the existence of the calcareous tetrahedra, and made in other cases (lens of vertebrate eye, nerves and muscles, &c.) the existence of formative stimuli very probable. Many off. Entw.-Mech.; O. Hertwig, Papers in Arch. f. mikr. Anat., "Die Zelle the facts generally known as functional adaptation (functionelle Anpassung-Roux) in botany and zoology may also belong to this category, i.e. be the effects of some external stimulus, but they are far from having been analysed in a satisfactory manner. That the structure of parts of the vertebrate skeleton is always in relation to their function, even under abnormal conditions, is well known; what is the real cause of differentiation in this case is difficult to say. Specific characters. EMDEN, a maritime town of Germany, in the Prussian It is obvious that we cannot answer the question why the province of Hanover, near the mouth of the Ems, 49 m. N.W. different ontogenetic effects are just what they are. Develop- from Oldenburg by rail. Pop. (1885) 14,019; (1905) 20,754. mental physiology takes the specific nature of form for The Ems once flowed beneath its walls, but is now 2 m. distant, granted, and it may be left for a really rational theory and connected with the town by a broad and deep canal, divided of the evolution of species in the future to answer into the inner (or dock) harbour and the outer (or "free port") the problem of species, as far as it is answerable at harbour. The latter is m. in length, has a breadth of nearly all. What we intend to do here is only to say in a few words 400 ft., and since the construction of the Ems-Jade and Dortwherein consists the specific character of embryonic organs. That mund-Ems canals, has been deepened to 38 ft., thus allowing embryonic parts are specific or typical in regard to their proto- the largest sea-going vessels to approach its wharves. The town plasm is obvious, and is well proved by the fact that the different is intersected by canals (crossed by numerous bridges), which parts of the embryo react differently to the same chemical or bring it into communication with most of the towns in East other reagents (Herbst, Loeb). That they may be typical also in Friesland, of which it is the commercial capital. The waterways regard to their nuclei was shown by Boveri for the generative which traverse and surround it and the character of its numerous cells of Ascaris; we are not able at present to say anything gabled medieval houses give it the appearance of an old Dutch, definite about the importance of this fact. The specific nature of rather than of a German, town. Of its churches the most notean embryonic organ consists to a high degree in the number worthy are the Reformed "Great Church" (Grosse Kirche), of cells composing it; it was shown for many cases that this a large Gothic building completed in 1455, containing the tomb number, and also the size of cells, is constant under constant of Enno II. (d. 1540), count of East Friesland; the Gasthausconditions, and that under inconstant conditions the number kirche, formerly the church of a Franciscan friary founded in is variable, the size constant; for instance, embryos which have 1317; and the Neue Kirche (1643-1647). Of its secular builddeveloped from one of the two first blastomeres show only half | ings, the Rathaus (town-hall), built in 1574-1576, on the model the normal number of cells in their organs (Morgan, Driesch). of that of Antwerp, with a lofty tower, and containing an interestWe have learnt that the successive steps of embryonic develop-ing collection of arms and armour, iş particularly remarkable. ment are to be regarded as effects, caused by stimuli, which partly There are numerous educational institutions, including classical exist in the embryo itself. But it must be noted that and modern schools, and schools of commerce, navigation not every part of the embryo is dependent on every and telegraphy. The town has two interesting museums. other one, but that there exists a great independence Emden is the seat of an active trade in agricultural produce and of the parts, to a varying degree in every case. This live-stock, horses, timber, coal, tea and wine. The deep-sea partial independence has been called self-differentiation | fishing industry of the town is important, the fishing fleet in 1902 (Selbstdifferenzierung) by Roux, and is certainly a characteristic feature of ontogeny. At the same time it must not be forgotten that the word is only relative, and that it only expresses our recognition of a negation. Self-differentiation. For instance, we know that the ectoderm of Echinus may develop further if the endoderm is taken away; in other words, that it develops by self-differentiation in regard to the endoderm, that its differentiation is not dependent on the endoderm; but it would be obviously more important to know the factors on which this differentiation is actually dependent than to know one factor on which it is not. The same is true for all other | experiments on" self-differentiation," whether analytical (Loeb, Schaper, Driesch) or not (grafting experiments, Born, Joest, &c.). Can we understand differentiation by means of the laws of natural phenomena offered to us by physics and chemistry? Most people would say yes, though not yet. Driesch has tried to show that we are absolutely not able to understand development, at any rate one part of it, i.e. the localization of the various successive steps of differentiation. But it is impossible to give any idea of this argument in a few words, and we can only say here that it is based on the experi Vitalism. numbering 67 vessels. Machinery, cement, cordage, wire ropes, tobacco, leather, &c. are manufactured. Emden is also of importance as the station of the submarine cables connecting Germany with England, North America and Spain. It has a regular steamboat service with Borkum and Norderney. Emden (Emuden, Emetha) is first mentioned in the 12th century, when it was the capital of the Eemsgo (Emsgau, or county of the Ems), one of the three hereditary countships into which East Friesland had been divided by the emperor. In 1252 the countship was sold to the bishops of Münster; but their rule soon became little more than nominal, and in Emden itself the family of Abdena, the episcopal provosts and castellans, established their practical independence. Towards the end of the 14th century the town gained a considerable trade owing to the permission given by the provost to the pirates known as "Viktualienbrüder" to make it their market, after they had been driven out of Gothland by the Teutonic Order. In 1402, after the defeat of the pirates off Heligoland by the fleet of Hamburg, Emden was besieged, but it was not reduced by Hamburg, with the aid of Edzard Cirksena of Greetsyl, until 1431. The town was held jointly by its captors till 1453, when Hamburg sold |