mass of finely nucleate corpuscles; amongst which the qualities of the parent embryo-cell, due to impregnation, are thus equally distributed.

The eight-fold cleavage of the yolk has been observed three days after impregnation in the Rab

bit, four days in the Guineapig, and ten days in the Bitch: always in ova toward the uterine end of the Fallopian tube.

In the Bitch the smooth surface of the zona pellucida becomes irregularly flocculent, as if a granulo-mucous substance had been deposited thereon in the Rabbit the ovum acquires a thick adventitious layer of albumen, fig. 565, a, before entering the uterus: in the Guinea-pig the zona continues smooth; and,

565

after entering the uterus, on the fourth day, it grows fainter as the mulberry state of the yolk is there attained, and it disappears when the germ-mass is completed. The act of impregnation being thus consummated, ulterior changes with manifold modifications attend the development of the ovum in different Mammalia.

§ 399. Development of Monotremata.-The ripe ovarian ovum, though large in proportion to that in higher, especially placental, Mammals, is very much less than in Birds or Reptiles. Its external coat is thick, smooth, highly refracting-a true zona pellucida': the germinal vesicle is th of an.inch in diameter: the larger proportion of vitelline matter, rich in granules and oil globules, is the chief distinctive character of the monotrematous ovum as a Mammalian one. I found two ovisacs with such mature ova in the left ovary of a female Ornithorhynchus, killed in September. In a specimen killed on the 6th of October (Yas River, New South Wales), the left ovary presented two discharged and altered ovisacs. The ova from these were situated at the upper part of the left uterus, and at the distance of about a line from each other. Each was spherical, and measured two lines and a half in diameter; the germ-mass, originally pale, had deepened to a yellow colour in the preserving liquor. The outer tunic had received no adventitious covering, but retained its smooth and polished exterior, and had not contracted any adherence to the uterine parietes. Each ovum was

imbedded in the soft, thick, plicated, smooth-surfaced, and wellorganised lining membrane of the uterus. In a second Ornithorhynchus, shot in the same locality, on the 7th of October, the ova, fig. 566, c, c, from the two discharged ovisacs, ib. b, b, were

566

situated a little below the middle of the left uterus; they were also spherical, each three lines in diameter, of a lighter colour than the preceding, specially at the upper part, from the subsidence of the contained vitelline or germinal mass: they were smooth, and rolled freely out of the position where they were lodged. In a third specimen, shot on the

evening on which the first specimen was obtained, the uterine ovum had the same spherical form, smooth exterior surface, and freedom from connexion with the uterus; but was of a lighter colour, owing to the increased quantity of its fluid contents, to which its greater size was chiefly attributable. It measured three lines and a half in diameter, and was situated in a depression or cell a little below the middle of the left uterus. The lining membrane of the uterus was much thickened and highly vascular in each of the above specimens. In all these ova the contents were of two kinds, viz. a greyish sub-transparent fluid, and a yellowish denser mass, which varied in their relative proportions as above-mentioned in the largest ovum, the yellow mass, germ or yolk, occupied about one-third of its cavity, while in the smallest it constituted four-fifths of the whole mass. The membrane, which may be the hyalinion or zona pellucida' of the ovarian ovum, but which I would still, as in 1834, call chorion,' offers a moderate degree of resistance when torn open, and yields equally in every direction when separated from the yolk, the rent margins curling inwards like the coat of an hydatid. This membrane is of a dull greyish colour, inclining to brown, slightly transparent, and more polished upon its inner than upon its outer surface. The fluid,

6

This term signifies the outer tunic' of the uterine ovum: it may be 'zona' or something laid upon the zona, or something superseding the zona, such as the animal layer of the blastoderina, or the outer or vascular layer of the allantois.

567

answering to that which appears between the yolk and zona pellucida after impregnation in the Rabbit's ovum (fig. 562, marked by the arrows), occupies a situation analogous to that of the albumen in the egg of the fowl, but had not become coagulated by the action of the spirit in which it had been so long immersed it divides the chorion, fig. 567, a, from the vitelline membrane, ib. b: this membrane, fig. 568, a, is thin, smooth, and transparent; adherent to parts of its inner surface was a thicker granular layer, answering to the 'blastoderm,' or germinal stratum, fig. 568, b. In each of the above impregnated Monotremes the discharged ovisacs, fig. 566, b, b, were of an elongate flaskshaped form, about three lines in length, and two in diameter, with the margins of the orifice, through which the ovum and granular substance had passed, everted, with a slight contraction, resembling the neck of a flask, below the aperture. On compressing these ovisacs, small portions of coagulated substance escaped. When longitudinally divided, they were found to consist of the same parts as the ovisac before impregnation; but the theca, or innermost parietes of the sac, was much thickened, and encroached irregularly upon the empty space, so as to leave only a cylindrical passage to the external opening.

Uterine Ovum, magnified and dissected,
Ornithorhynchus. LXXVII'.

On the 8th of December Dr. Bennett discovered in the subterranean nest of an Orni

568

Portion of the vitelline membrane and germinal stratum, Ornithorhynchus. LXXVII'.

thorhynchus three living young, naked, not quite two inches in length, fig. 600. On the 12th of August (1864) a female Echidna hystrix was captured in the hollow of a prostrate 'cotton tree,' in Colac Forest, Victoria Province, Australia, having a young one, fig. 603, e, with its head buried in a mammary or marsupial fossa, ib. c. This young one was naked, of a bright red colour, and one inch two lines in length. Between the condition of the uterine ovum, as in fig. 567, and that of the (probably new-born, or recently born) young Monotremes, abovementioned, I have not hitherto received materials for further elucidating the development of the foetus in this singular group of Mammals: whether cleavage of the yolk takes place prior to the

1 LXXVII'. I was indebted to my old friend and fellow-student, GEORGE BENNETT, now F.R.S, for the above mentioned specimens.

entry of the ovum into the uterus still remains a matter for observation. The young of both Ornithorhynchus and Echidna will be described in the chapter on the Mammary organs.

§ 400. Development of Marsupialia.-On the 27th of August (1833), a female Kangaroo (Macropus major), captive in the Gardens of the London Zoological Society, received the male. She stood with her fore-paws off the ground; the male mounted, more canino, embracing her neck with his fore-paws, and retained his hold during a full quarter of an hour: during this period the coitus was repeated three times, and on the second occasion much fluid escaped from the vulva. The male was removed from the female in the evening of the same day, and was not afterwards admitted to her. On September the 2nd, six days after the coitus, I examined the pouch of the female; and this scrutiny was repeated every morning and evening until the birth of the young Kangaroo had taken place. It happened in the night of October 4, thirty-eight days after the coitus. On the morning of the 5th of October, I found the young in the pouch, pendant from the tip of the left upper nipple, of the size and shape shown in fig. 606 it will be described in a subsequent chapter.

140

The ovarian ovum, in the Kangaroo, agrees in all essential points with that of placental Mammalia: the main modification is the greater proportion of vitelline substance, and the smaller proportion of the surrounding fluid in the ovisac. In a female Macropus Parryi, the ovum from the largest ovisac of the left ovarium measuredth of a line in diameter, the germinal vesicle Tth of a line in diameter. We are at present ignorant of the changes that take place in the development of the ovum between the period of impregnation until about the twentieth day of uterine gestation. At this time, in the great Kangaroo (Macropus major), the uterine fœtus, fig. 537, measures eight lines from the mouth to the root of the tail; the gape of the mouth is wide; the tongue large and protruded, fig. 569; the nostrils are small round apertures; the eyeball is not yet wholly defended by the palpebral folds; the visceral cleft reduced to the meatus auditorius externus is not provided with an auricle; a posterior cervical fissure was either unclosed, or the delicate cicatrix had given way in the manipulation of the fœtus. The fore-extremities are the largest and strongest; they are each terminated by five well-marked digits; those of the hind legs are not yet developed. The tail is two lines long, thick and strong at the commencement; impressions of the ribs are visible at the sides of the body the membranous tube of the spinal marrow may be

traced along the back between the ununited elements of the vertebral arches; posterior to the umbilical cord there is a small projecting penis, and behind that, on the same prominence, is the anus. This foetus and its appendages were enveloped in a large chorion, ib. i, puckered up into numerous folds, some of which were insinuated between folds of the vascular lining membrane of the uterus, but the greater portion was collected into a wrinkled mass. The entire ovum was removed without any opposition from a placental or villous adhesion to the uterus. The chorion, fig. 567, a, a, was extremely thin and lacerable, and showed no trace of villi on the outer surface. The membrane, ib. b., extending from the umbilicus to the inner surface of the chorion, was highly vascular. The foetus was immediately enveloped in a transparent amnios. On turning the chorion away from the fœtus, it was found to adhere to the vascular membrane; but they could be separated from each other, without laceration, for the extent of an inch; at this distance from the umbilicus the adhesion was closer: and here the umbilical membrane terminated in a well-defined ridge, formed by the trunk of a bloodvessel. When spread out, as at b, b, fig. 569, its figure was that of a cone, of which the apex was the umbilical cord, and the base the vena terminalis.' Three vessels diverged from the umbilical cord and ramified over it. Two were continuations of the terminal or marginal vein: the third was the arterial trunk. The amnios, ib. c, was reflected from the umbilical cord, and formed, as usual, the immediate investment of the foetus.

The umbilical cord measured two lines in length and one in diameter: besides the three vessels above-mentioned, it included a small loop of intestine; and from the extremity of the latter a filamentary process was continued to the vascular membrane. On tracing the contents of the cord into the abdomen, the two larger vessels, filled with coagulated blood, were found to unite; the common trunk then passed backward beneath the duodenum, and after being joined by the mesenteric vein, went to the under surface of the liver, where it penetrated that viscus: this was consequently an omphalo-mesenteric or vitelline vein. The artery was a branch of the mesenteric. The membrane, therefore, upon which they ramified answered to the vitellicle, i. e. the vascular and mucous layers of the germinal membrane, which spreads over the yolk in oviparous animals, and which constitutes the so called umbilical vesicle' of the embryo of placental Mammalia. The filamentary pedicle which connected this membrane to the intestine was given off near the end of the ileum.