In neither of the above foetuses of the Kan garoo was there any corresponding trace of umbilical vein, although there was a distinct ligamentum suspensorium hepatis, formed by a duplicature of the peritoneum descending from the diaphragm to the notch lodging the gall-bladder, and not entering, as usual, the fissure to the left of that notch : the allantois is too small, and its function too limited for the preservation of any permanent trace of its peculiar vein.
The small intestines in the mammary foetus, one inch and a half long, when compared with those of the uterine fcetus above de scribed, were found to have acquired several additional convolutions ; the fold to which the umbilical vesicle had been attached was still distinct, but now drawn in to the back of the abdomen. The ccecum was much elongated, but the colon proportionately not more deve loped than in the uterine fcetus ; the subse quent modification, therefore, of the large in testines seems evidently destined to complete the digestion of the vegetable food.
The stomach was not sacculated, but the division between the cardiac and middle com partments was more marked than in the uterine foetus. The liver had now advanced in its development beyond the oviparous form which it presented in the uterine fcetus, the right lobe being subdivided into three. The supra-renal glands bore the same proportionate size to the kidneys. The testes were still larger than the kidneys, and were situated below them, not having yet passed out of the abdomen : this takes place when the mammary fcetus is about three inches long from the nose to the root of the tail. The ductus arteriosus was distinct in the small mammary fcetus, but I could not perceive any trace of the thymus gland. Is this gland unnecessary on account of the pre cocious development of the lungs ? or because of the small size and gradual growth of the brain ? The latter appears the more probable condition of its absence, as in the ovovivipa rous classes with small and simple brains the thymus gland is rudimental or of doubtful existence.
Notwithstanding that the new-born Kanga roo possesses greater powers of action than the same sized embryo of a sheep, and approxi mates more nearly in this respect to the new born young of the rat, yet it is evidently in ferior to the latter. For, although it is enabled by the muscular power of its lips to grasp and adhere firmly to the nipple, it seems to be unable to draw sustenance therefrom by its own unaided efforts. The mother, as Professor Geoffroy and Mr. Morgan have shown, is therefore provided with the peculiar adaptation of a muscle (analogous to the cremaster) to the mammary gland, for the evident purpose of injecting the milk from the nipple into the mouth of the adherent fcetus. Now it can
scarcely be supposed that the foetal efforts of suction should always be coincident with the maternal act of injection ; and if at any time this should not be the case, a fatal accident might happen from the milk being forcibly injected into the larynx, unless that aperture were guarded by some special contrivance. Professor Geoffroy first described the modi fication by which this purpose is effected ; and Mr. Hunter appears to anticipated the ne cessity for such a structure, for he dissected two small mammary fetuses of the Kangaroo for the especial purpose of showing the rela tion of the larynx to the posterior nares. The epiglottis and arytenoid cartilages are elongated and approximated, and the rima glottidis 'is thus situated at the apex of a cone-shaped larynx, (fig. 142, c,) which projects, as in the Cetacea, into the posterior nares, where it is closely embraced by the muscles of the soft palate. The air-passage (b) is thus completely separated from the fauces, and the injected milk passes in a divided stream on either side the larynx to the cesonhagns.
Thus aided and protected by modifications of structure, both in the system of the mother and its own, designed with especial reference to each other's peculiar condition, and afford ing, therefore, the most irrefragable evidence of creative foresight, the small offspring of the Kangaroo continues to increase, from suste nance exclusively derived from the mother, for a period of about eight months. During this period the hind legs and tail assume a great part of their adult proportions; the muzzle elongates; the external ears and eyelids are completed ; the hair begins to be developed at about the sixth month. At the eighth month the young Kangaroo may be seen frequently to protrude its head from the mouth of the pouch, and to crop the grass at the same time that the mother is browsing. Having thus acquired additional strength, it quits the pouch and hops at first with a feeble and vacillating gait, but continues to return to the pouch for occasional shelter and supplies of food till it has attained the weight of ten pounds. After this it will occasionally insert its head for the pur pose of sucking, notwithstanding another fmtus may have been deposited in the pouch ; for the latter, as we have seen, attaches itself to a dif ferent nipple from the one which had been pre viously in use.