(ib., 1867), and Church Vestments (ib., 1868); M. Dreger, Künst- | concluded that embryology would exclude the study of larvae, lerische Entwicklung der Weberei und Stickerei (Vienna, 1904); in which the whole or the greater part of the development takes Madame I. Errera, Collection de broderies anciennes (Brussels, 1905); L. de Farcy, La Broderie (Paris, 1890); R. Forrer, Die Gräber und Textilfunde von Achmim-Panopolis (Strassburg, 1891); F. R. Fowke, The Bayeux Tapestry (London, 1898); Rev. C. H. Hartshorne, On English Medieval Embroidery (ib., 1848); M. B. Huish, Samplers and Tapestry Embroideries (ib., 1900); A. F. Kendrick, English Embroidery (ib., 1905); English Embroidery executed prior to the Middle of the 16th Century (Burlington Fine Arts Club Exhibition, 1905, introduction by A. F. Kendrick); E. Lefebure, Embroideries and Lace, translated by A. S. Cole, C.B. (London, 1888); F. Marshall, Old English Embroidery (ib., 1894); E. M. Rogge, Moderne KunstNadelarbeiten (Amsterdam, 1905); South Kensington Museum, Catalogue of Special Loan Exhibition of Decorative Art Needlework (1874); W. G. P. Townshend, Embroidery (London, 1899). For further examples of ecclesiastical embroidery see the articles CHASUBLE, COPE, DALMATIC and MITRE. (A. F. K.; A. S. C.) EMBRUN, a town in the department of the Hautes Alpes in S.E. France. It is built at a height of 2854 ft. on a plateau

that rises above the right bank of the Durance. It is 27 m. by rail from Briançon and 24 m. from Gap. Its ramparts were demolished in 1884. In 1906 the communal pop. (including the garrison) was 3752. Besides the Tour Brune (11th century) and the old archiepiscopal palace, now occupied by government offices, barracks, &c., the chief object of interest in Embrun is its splendid cathedral church, which dates from the second half of the 12th century. Above its side door, called the Réal, there existed till 1585 (when it was destroyed by the Huguenots) a fresco, probably painted in the 13th century, representing the Madonna: this was the object of a celebrated pilgrimage for many centuries. Louis XI. habitually wore on his hat a leaden image of this Madonna, for which he had a very great veneration, since between 1440 and 1461, during the lifetime of his father, he had been the dauphin, and as such ruler of this province. Embrun was the Eburodunum or Ebredunum of the Romans, and the chief town of the province of the Maritime Alps. The episcopal see was founded in the 4th century, and became an archbishopric about 800. In 1147 the archbishops obtained from the emperor Conrad III. very extensive temporal rights, and the rank of princes of the Holy Roman Empire. In 1232 the county of the Embrunais passed by marriage to the dauphins of Viennois. In 1791 the archiepiscopal see was suppressed, the region being then transferred to the diocese of Gap, so that the once metropolitan cathedral church is now simply a parish church. The town was sacked in 1585 by the Huguenots and in 1692 by the duke of Savoy. Henri Arnaud (1641-1721), the Waldensian pastor and general, was born at Embrun.

See A. Albert, Histoire du diocèse d'Embrun (2 vols., Embrun, 1783); M. Fornier, Histoire générale des Alpes Maritimes ou Cottiennes et particulière de leur métropolitaine Embrun (written 1626-1643), published by the Abbé Paul Guillaume (3 vols., Paris and Gap, 1890-1891); A. Fabre, Recherches historiques sur le pèlerinage des rois de France à N. D. d'Embrun (Grenoble, 1859); A. Sauret, Essai historique sur la ville d'Embrun (Gap, 1860). (W. A. B. C.) EMBRYOLOGY. The word embryo is derived from the Gr. Eußpvov, which signified the fruit of the womb before birth. In its strict sense, therefore, embryology is the study of the intrauterine young or embryo, and can only be pursued in those animals in which the offspring are retained in the uterus of the mother until they have acquired, or nearly acquired, the form of the parent. As a matter of fact, however, the word has a much wider application than would be gathered from its derivation. All animals above the Protozoa undergo at the beginning of their existence rapid growth and considerable changes of form and structure. During these changes, which constitute the development of the animal, the young organism may be incapable of leading a free life and obtaining its own food. such cases it is either contained in the body of the parent or it is protruded and lies quiescent within the egg membranes; or it may be capable of leading an independent life, possessing in a functional condition all the organs necessary for the maintenance of its existence. In the former case the young organism is called an embryo, in the latter a larva. It might thus be In the mammalia the word foetus is often employed in the same signification as embryo; it is especially applied to the embryo in the later stages of uterine development.

In

place outside the parent and outside the egg. But this is not
the case; embryology includes not only a study of embryos
as just defined, but also a study of larvae. In this way the
scope of the subject is still further widened. As long as em-
bryology confines its attention to embryos, it is easy to fix its
limits, at any rate in the higher animals. The domain of em-
bryology ceases in the case of viviparous animals at birth, in
the case of oviparous animals at hatching; it ceases as soon as
the young form acquires the power of existing when separated
from the parent, or when removed from the protection of the egg
membranes. But as soon as post-embryonic developmental
changes are admitted within the scope of the subject, it becomes
on close consideration difficult to limit its range. It must include

all the developmental processes which take place as a result of
sexual reproduction. A man at birth, when he ceases to be an
embryo, has still many changes besides those of simple growth
to pass through. The same remark applies to a young frog
at the metamorphosis. A chick even, which can run about
and feed almost immediately after hatching, possesses a plumage
very different from that of the full-grown bird; a starfish at
the metamorphosis is in many of its features quite different
from the form with which we are familiar. It might be attempted
to meet this difficulty by limiting embryology to a study of all
those changes which occur in the organism before the attainment
But this merely shifts the difficulty to
of the adult state.
another quarter, and makes it necessary to define what is meant
doubt it is not difficult when man and the higher animals alone
by the adult state. At first sight this may seem easy, and no
are in question, for in these the adult state may be defined
comparatively sharply as the stage of sexual maturity. After
that period, though changes in the organism still continue, they
are retrogressive changes, and as such might fairly be excluded
from any account of development, which clearly implies progres-
sion, not retrogression. But, as so often happens in the study
of organisms, formulae which apply quite satisfactorily to one
group require modifications when others are considered. Does
sexual maturity always mark the attainment of the adult state?
Is the Axolotl adult when it acquires its reproductive organs?
Can a larval Ctenophore, which acquires functional reproductive
glands and still possesses the power of passing into the form
ordinarily described as adult in that group, be considered to have
reached the end of its development? Or-to take the case of
those animals, such as Amphioxus, Balanoglossus, and many
segmented worms in which important developmental processes
occur, e.g. formation of new gill slits, of gonadial sacs, or even of
whole segments of the body, long after the power of reproduction
has been acquired-how is the attainment of the adult state

to be defined, for it is clear that in them the attainment of sexual
maturity does not correspond with the end of growth and
development? If, then, embryology is to be regarded as includ-
ing not only the study of embryos, but also that of larvae, i.e.
if it includes the study of the whole developmental history of
the individual—and it is impossible to treat the subject rationally
unless it is so regarded-it becomes exceeding difficult to fix
any definite limit to the period of life with which embryology
concerns itself. The beginning of this period can be fixed, but
not the end, unless it be the end of life itself, i.e. death. The
science of embryology, then, is the science of individual develop-
ment, and includes within its purview all those changes of form
and structure, whether embryonic, larval or post-larval, which
characterize the life of the individual. The beginning of this
period is precise and definite-it is the completion of the fertiliza-
tion of the ovum, in which the life of the individual has its start.
The end, on the other hand, is vague and cannot be precisely
defined, unless it be death, in which case the period of life with
which embryology concerns itself is coincident with the life of
the individual. To use the words of Huxley ("Cell Theory,"
Collected Works, vol. i. p. 267): “Development, therefore, and
life are, strictly speaking, one thing, though we are accustomed
to limit the former to the progressive half of life merely, and to

FIG. 17.-LINEN PRAYER CARPET, QUILTED AND EMBROIDERED IN CHAIN STITCH WITH COLOURED SILKS, CHIEFLY WHITE, YELLOW,
GREEN AND RED
The border consists of a wide band set between two narrow ones, each with a waved continuous stem with blossoms in the wavings. Similar floral scrolling and leafy stem
ornament fills the space beyond the pointed shape at the upper end, which is edged with acanthus leaf devices. The main ground below the niche or pointed shape is a blossoming
plant, with balanced bunches of flowers between which are leaves, formally arranged in a pointed oval shape. Persian work, 18th century, 4 ft. 6 in. x 2 ft. 11 in.
Albert Museum.)
(Victoria and

FIG. 18.---PART OF A SICILIAN COVERLET, OF THE END OF THE 14TH CENTURY.

It is of white linen, quilted and padded in wool so as to throw the design into relief. The scenes represented, taken from the Story of Tristan, with inscriptions in the Sicilian dialect, are as follows:-(1) COMU: LU AMOROLDU FA BANDIRI: LU OSTI: IN CORNUUALGIA (How the Morold made the host to go to Cornwall); (2) COMU: LU RRE: LANGUIS: CUMANDA: CHI UAIA: LO OSTI: CORNUAGLIA (How King Languis ordered that the host should go to Cornwall; (3) COMU: LU RRE: LANGUIS: MANDA: PER LU TRABUTU IN CORNUALIA (How King Languis sent to Cornwall for the tribute); (4) COMU: (li m) ISSAGIERI: So UINNTI: AL RRE: MARCU: PER LU TRIBUTU DI SECTI ANNI (How the ambassadors are come to King Mark for the tribute of seven years); (5) COMU: LU AMOROLDU UAI: IN CORNUUALGIA (How the Morold comes to Cornwall); (6) COMU: LU AMOROLDU: FA SULDARI: LA GENTI (How the Morold made the people pay); (7) COMU: T(RISTAINU): DAI: LU GUANTU ALLU AMOROLDU DELA BACTAGLIA (How Tristan gives the glove of battle to the Morold); (8) COMU: LU AMOROLDU: E UINUTU: IN CORNUUALGIA: CUM XXXX GALEI: (How the Morold is come to Cornwall with forty galleys); (9) COMU TRISTAINU BUCTA: LA UARCA: ARRETU: INTU: ALLU MARU (How Tristan struck his boat behind him into the sea); (10) COMU: TRISTAINU: ASPECTA: LU AMOROLDU: ALLA ISOLA DI LU MARU: SANSA UINTURA (How Tristan awaits the Morold on the isle Sanza Ventura in the sea); (11) COMU: TRISTAINU FERIU LU AMOROLLDU IN TESTA (How Tristan wounded the Morold in the head); (12) COMU: LU INNA (?) DELU AMOROLDU: ASPECTTAUA LU PATRUNU (How the Morold's page (?) awaited his master); (13) COMU LU AMORODU FERIU: TRISTAINU A TRADIMANTU (How the Morold wounded Tristan by treachery); (14) ... SITA: IN AIRLANDIA (... in Ireland).

speak of the retrogressive half as decay, considering an imaginary | are evolved, and the zygote remains apparently unchanged; resting-point between the two as the adult or perfect state." There are two kinds of reproduction, the sexual and the asexual. The sexual method has for its results an increase of the number of kinds of individual or organism, whereas Reproduc the asexual affords an increase in the number of tion. individuals of the same kind. If the asexual method of reproduction alone existed, there would, so far as our knowledge at present extends, be no increase in the number of kinds of organism: no new individuality could arise. The first establishment of a new kind of individual by the sexual process is effected in a very similar manner in all Metazoa. The parent produces by a process of unequal fission, which takes place at a part of the body called the reproductive gland, a small living organism called the reproductive cell. There are always two kinds of reproductive cells, and these are generally produced by different animals called the male and female respectively (when they are produced by the same animal it is said to be hermaphrodite). The reproductive cell produced by the male is called the spermatozoon, and that produced by the female, the ovum. These two organisms agree in being small uninucleated masses of protoplasm, but differ considerably in form. They are without the organs of nutrition, &c., which characterize their parents, but the ovum nearly always possesses, stored up within its protoplasm, a greater or less quantity of vitelline matter or food-yolk, while the spermatozoon possesses in almost all cases the power of locomotion. The object with which these two minute and simple organisms are produced is to fuse with one another and give rise to one resultant uninucleated (for the nuclei fuse) organism or cell, which is called the zygote. This process of fusion between the two kinds of reproductive cells, which are termed gametes, is called conjugation: it is the process which is sometimes spoken of as the fertilization of the ovum, and its result is the establishment of a new individual. This new individual at first is simply a uninucleated mass of living matter, which always contains a certain amount of food-yolk, and is generally bounded by a delicate cuticular membrane called the vitelline membrane. In form the newly established zygote resembles the female gamete or ovum-so much so, indeed, that it is frequently called the ovum; but it must be clearly understood that although the bulk of its matter has been derived from the ovum, it consists of ovum and spermatozoon, and, as shown by its subsequent behaviour, the spermatozoon has quite as much to do with determining its vital properties

(2) the acquisition of the properties which constitutes the growth and development of the organism proceeds in a perfectly definite sequence, which, so far as is known, cannot be altered; (3) just as the features of the growing organism change under the continued action of the external conditions, so the external conditions themselves must change as the organism is progressively evolved. With regard to this last change, it may be said generally that it is usually, if not always, effected by the organism itself, making use of the properties which it has acquired at earlier stages of its growth, and acting in response to the external conditions. There is, to use a phrase of Mr Herbert Spencer, a continuous adjustment between the external and internal relations. For every organism a certain succession of conditions is necessary if the complete and normal evolution of properties is to take place. Within certain limits, these conditions may vary without interfering with the normal evolution of the properties, though such variations are generally responded to by slight but unimportant variation of the properties (variation of acquired characters). But if the variation of the conditions is too great, the evolved properties become abnormal, and are of such a nature as to preclude the normal evolution of the organism; in other words, the action of the conditions upon the organism is injurious, causing abortions and, ultimately, death. For many organisms the conditions of existence are well known for all stages of life, and can be easily imitated, so that they can be reared artificially and kept alive and made to breed in confinement-e.g. the common fowl. But in a large number of cases it is not possible, through ignorance of the proper conditions, or on account of the difficulty of imitating them, to make the organism evolve all its properties. For instance, there are many marine larvae which have never been reared beyond a certain point, and there are some organisms which, even when nearly full-grown-a stage of life at which it is generally most easy to ascertain and imitate the natural conditions-will not live, or at any rate will not breed, in captivity. Of late years some naturalists have largely occupied themselves with experimental observation of the effects on certain organisms of marked and definite changes of the conditions, and the name of Developmental Mechanics (or Physiology of Development) has been applied to this branch of study (see below).

as the ovum.

To the unaided eye the main difference between the newly formed zygotes of different species of animals is that of bulk, and this is due to the amount of food-yolk held in suspension in the protoplasm. The ovum of the fowl is 30 mm. in diameter, that of the frog 1.75 mm., while the ova of the rabbit and Amphioxus have a diameter of 1 mm. The food-yolk is deposited in the ovum as a result of the vital activity of its protoplasm, while the ovum is still a part of the ovary of the parent. It is an inert substance which is used as food later on by the developing embryo, and it acts as a dilutant of the living matter of the ovum. It has a profound influence on the subsequent developmental process. The newly formed zygotes of different species of animals have undoubtedly, as stated above, a certain family resemblance to one another; but however great this superficial resemblance may be, the differences must be most profound, and this fact becomes at once obvious when the properties of these remarkable masses of matter are closely investigated.

Causes of

develop

ment.

As in the case of so many other forms of matter, the more important properties of the zygote do not become apparent until it is submitted to the action of external forces. These forces constitute the external conditions of existence, and the properties which are called forth by their action are called the acquired characters of the organism. The investigation of these properties, particularly of those which are called forth in the early stages of the process, constitutes the science of Embryology. With regard to the manifestation of these properties, certain points must be clearly understood at the outset :-(1) If the zygote is withheld from the appropriate external influences, e.g. if a plant-seed be kept in a box free from moisture or at a low temperature, no properties

Gametogeny.

In normal fertilization, as a rule, only one spermatozoon fuses with the ovum. It has been observed in some eggs that a membrane, formed round the ovum immediately after the entrance of the spermatozoon, prevents the entrance of others. If more than one spermatozoon enters, a corresponding number of male pronuclei are formed, and the subsequent development, if it takes place at all, is abnormal and soon ceases. An egg by ill-treatment (influence of chloroform, carbonic acid, &c.) can be made to take more than one spermatozoon. In some animals it appears that several spermatozoa may normally enter the ovum (some Arthropoda, Selachians, Amphibians and Mammals), but of these only one forms a male pronucleus (see below), the rest being absorbed. Gametogeny is the name applied to the formation of the gametes, i.e. of the ova and spermatozoa. The cells of the reproductive glands are the germ cells (oögonia, spermatogonia). They undergo division and give rise to the progametes, which in the case of the female are sometimes called oocytes, in the case of the male spermatocytes. The oocytes are more familiarly called the ovarian ova. The nucleus of the oocyte is called the germinal vesicle. The oocyte (progamete) gives rise by division to the ovum or true gamete, the nucleus of which is called the female pronucleus. As a general rule the oocyte divides unequally twice, giving rise to two small cells called polar bodies, and to the ovum. The first formed polar body frequently divides when the oöcyte undergoes its second and final division, so that there are three polar bodies as well as the ovum resulting from the division of the oöcyte or progamete. Sometimes the ovum arises from the oocyte by one division only, and there is only one polar body (e.g. mouse, Sobotta, Arch. f. mikr. Anat., 1895, p. 15). The polar bodies are oval, but as a rule they are so small as to be incapable of fertilization. They (see Francotte, Bull. Acad. Belg. (3), xxxiii., 1897, p. 278), the may therefore be regarded as abortive ova. In one case, however first formed polar body is nearly as large as the ovum, and is sometimes fertilized and develops. The spermatogonia are the cells of the testis; these produce by division the spermatocytes (progametes), which divide and give rise to the spermatids. In most cases which have been investigated the divisions by which the spermatids arise from the spermatocytes are two in number, so

tion.

that each spermatocyte gives origin to four spermatids. Each an altered environment. For instance, it has been asserted that spermatid becomes a functional spermatozoon or male gamete. the addition of a certain quantity of chloride of magnesium and The gametogeny of the male therefore closely resembles that of the other substances to sea-water will cause the unfertilized ova of female, differing from it only in the fact that all the four products certain marine animals (Arbacia, Chaetopterus) to develop (J. of the progamete become functional gametes, whereas in the female Loeb, American Journal of Physiology, ix., 1901, p. 423); and only one, the ovum, becomes functional, the other three (polar bodies) according to M. Y. Delage (Comptes rendus, 135, 1902. Nos 15 and being abortive. In the spermatogenesis of the bee, however, the 16) such development may occur after the formation of polar bodies, spermatocyte only divides once, giving rise to a small polar-body-like the chromosomes undergoing reduction and the full number being structure and one spermatid (Meves, Anat. Anzeiger, 24, 1904, pp. regained in the segmenting stage. These experiments, if authenti29-32). The nucleus of the male gamete is not called the male pro- cated, suggest that ova have the power of development, but are not nucleus, as would be expected, that term being reserved for the able to exercise it in their normal surroundings. There is reason to second nucleus which appears in the ovum after fertilization. As believe that the same assertion may be made of spermatozoa. this is in all probability derived entirely from the nucleus of the Phenomena of the nature of parthenogenesis have never been obspermatozoon, we should be almost justified in calling the nucleus served in the male gamete, but it has been suggested by A. Giard of the spermatozoon the male pronucleus. In most forms in which (Cinquantenaire de la Soc. de Biol., 1900) that the phenomenon the formation of the gametes from the progamete has been accurately of the so-called fertilization of an enucleated ovum which has been followed, and in which the progamete of both sexes divides twice in described by T. Boveri and Delage in various eggs, and which results forming the gametes, the division of the nucleus presents certain in development up to the larval form (merogony), is in reality a case peculiarities. In the first place, between the first division and the in which the male gamete, unable to undergo development in second it does not enter into the resting state, but immediately ordinary circumstances on account of its small size and specialization proceeds to the second division. In the second place, the number of of structure has obtained a nutritive environment which enables chromosomes which appear in the final divisions of the progametes it to display its latent power of development. Moreover, A. M. Giard and assist in constituting the nuclei of the gametes is half the number suggests that in some cases of apparently normal fertilization one which go to constitute the new nuclei in the ordinary nuclear divisions of the pronuclei may degenerate, the resultant embryo being the of the animal. The number of chromosomes of the nucleus of the product of one pronucleus only. In this way he explains certain gamete is therefore reduced, and the divisions by which the gametes cases of hybridization in which the paternal (rarely the maternal) arise from the progametes are called reducing (maiotic) divisions. type is exclusively reproduced. For instance, in the batrachiate It is not certain, however, that this phenomenon is of universal Amphibia, Héron Royer succeeded in 1883 in rearing, out of a vast occurrence, or has the significance which is ordinarily attributed to it. number of attempts, a few hybrids between a female Pelobates In the parthenogenetic ova of certain insects, e.g. Rhodites rosae fuscus and a male Rana fusca; the product was a Rana fusca. (Henking), Nematus lacteus (Doncaster, Quart. Journal Mic. Science, He also crossed a female Bufo vulgaris with a male Bufo calamita; 49, 1906, pp. 561-589), reduction does not occur, though two polar in the few cases which reached maturity the product was obviously bodies are formed. a Bufo calamita. Finally, H. E. Ziegler (Arch. f. Ent.-Mech., 1898, As soon as the spermatozoon has conjugated with the ovum, a p. 249) divided the just-fertilized ovum of a sea-urchin in such a way second nucleus appears in the ovum. This is undoubtedly derived that each half had one pronucleus; the half with the male profrom the spermatozoon, possibly from its nucleus only, nucleus segmented and formed a blastula, the other degenerated. Fertiliza- and is called the male pronucleus. It possesses in the It is said that in a few species of animals males do not occur, and adjacent protoplasm a well-marked centrosome. The that parthenogenesis is the sole means of reproduction (a species of general rule appears to be that the female pronucleus is without Ostracoda among Crustacea; species of Tenthredinidae, Cynipidae a centrosome, and that no centrosome appears in the female in and Coccidae among Insecta); this is the thelytoky of K. T. E. von the divisions by which the gamete arises from the progamete. Siebold. The number of species in which males are unknown is If this is true, the centrosome of the zygote nucleus must be entirely constantly decreasing, and it is quite possible that the phenomenon derived from that of the male pronucleus. This accounts for the does not exist. Parthenogenesis, however, is undoubtedly of fact, which has been often observed, that the female pronucleus is frequent occurrence, and is of four kinds, namely, (1) that in which not surrounded by protoplasmic radiations, whereas such radiations males alone are produced, e.g. honey-bees (arrhenotoky); (2) that are present round the male pronucleus in its approach to the female. in which females only are produced (thelytoky), as in some saw-flies; In the mouse the subsequent events are as follow:-Both pronuclei (3) that in which both sexes are produced (deuterotoky), as in some assume the resting form, the chromatin being distributed over the saw-flies; (4) that in which there is an alternation of sexual and nuclear network, and the nuclei come to lie side by side in the centre parthenogenetic generations, as in Aphidae, many Cynipidae, &c. of the egg. A long loop of chromatin then appears in each nucleus It would appear that "parthenogenesis does not favour the proand divides up into twelve pieces, the chromosomes. The centrosome duction of one sex more than another, but it is clear that it decidedly now divides, the membranes of both nuclei disappear, and a spindle favours the production of a brood that is entirely of one sex, but is formed. The twenty-four chromosomes arrange themselves at the which sex that is differs according to circumstances" (D. Sharp, centre of this spindle and split longitudinally, so that forty-eight Cambridge Natural History, "Insects," pt. i. p. 498). chromosomes are formed. Twenty-four of these, twelve male and Insecta and Crustacea exceptional parthenogenesis occurs: a certain twelve female, as it is supposed, travel to each pole of the spindle proportion of the eggs laid are capable of undergoing either the whole and assist in giving rise to the two nuclei. At the next nuclear or a part of development parthenogenetically, e.g. Bombyx mori, &c. division twenty-four chromosomes appear in each nucleus, each of (A. Brauer, Arch. f. mikr. Anat., 1893; consult also E. Maupas on which divides longitudinally; and so in all subsequent divisions. parthenogenesis of Rotifera, Comp. rend., 1889-1891, and R. LauterThe fusion of the two pronuclei is sometimes effected in a manner born, Biol. Centralblatt, xviii., 1898, p. 173). slightly different from that described for the mouse. In Echinus, for instance, the two pronuclei fuse, and the spindle and chromosomes are formed from the zygote nucleus, whereas in the mouse the two pronuclei retain their distinctness during the formation of the chromosomes. There appears, however, to be some variation in this respect: cases have been observed in the mouse in which fusion of the pronuclei occurs before the separation of the chromosomes. Parthenogenesis, or development of the female gamete without fertilization, is known to occur in many groups of the animal kingdom. Attempts have been made to connect this phenomenon with peculiarities in the gametogeny. For genesis. instance, it has been said that parthenogenetic ova form only one polar body. But, as we have seen, this is sometimes the case in eggs which are fertilized, and parthenogenetic ova are known which form two polar bodies, e.g. ova of the honey-bee which produce drones (Morph. Jahrb. xv., 1889, p. 85). ova of Rotifera which produce males (Zool. Anzeiger, xx., 1897, p. 455), ova of some saw-flies and gall flies which produce females (L. Doncaster, Quart. Journ. Mic. Sc., 49, 1906, pp. 561-589). Again it has been asserted that in parthenogenetic eggs the polar bodies are not extruded from the ovum; in such cases, though the nucleus divides, those of its products which would in other cases be extruded in polar bodies remain in the protoplasm of the ovum. But this is not a universal rule, for in some cases of parthenogenesis polar bodies are extruded in the usual way (Aphis, some Lepidoptera), and in some fertilized eggs the polar bodies are retained in the ovum.

Partheno

It is quite probable that parthenogenesis is more common than has been supposed, and it appears that there is some evidence to show that ova, which in normal conditions are incapable of developing without fertilization, may yet develop if subjected to

In some

Determination of sex.

The question of the determination of sex may be alluded to here. Is sex determined at the act of conjugation of the two gametes? Is it, in other words, an unalterable property of the zygote, a genetic character? Or does it depend upon the conditions to which the zygote is subjected in its development? In other words, is it an acquired character? It is impossible in the present state of knowledge to answer these questions satisfactorily, but the balance of evidence appears to favour the view that sex is an unalterable, inborn character. Thus those twins which are believed to come from a split zygote are always of the same sex, members of the same litter which have been submitted to exactly similar conditions are of different sexes, and all attempts to determine the sex of offspring in the higher animals by treatment have failed. On the other hand, the male bee is a portion of a female zygote the queen-bee. The same remark applies to the male Rotifer, in which the zygote always gives rise to a female, from which the male arises parthenogenetically, but in these cases it does not appear that the production of males is in any way affected by external conditions (see R. C. Punnett, Proc. Royal Soc., 78 B, 1906, p. 223). It is said that in human societies the number of males born increases after wars and famines, but this, if true, is probably due to an affection of the gametes and not of the young zygote. For a review of the whole subject see L. Cuénot, Bull. sci. France et Belgique, xxxii., 1899, pp. 462535.

The first change the zygote undergoes in all animals is what is generally called the segmentation or cleavage of the ovum. This consists essentially of the division of the nucleus into a number of nuclei, around which the protoplasm sooner or later