This section is from the book "General Outline Of The Organization Of The Animal Kingdom, And Manual Of Comparative Anatomy", by Thomas Rymer Jones. Also available from Amazon: A General Outline of the Animal Kingdom and Manual of Comparative Anatomy.
* "On the Generation of Marsupial Animals, with a Description of the Impregnated Uterus of the Kangaroo," by Richard Owen, Esq., Phil. Trans. 1834.
(2443). The body of the foetus itself was immediately enclosed in a transparent membrane (b), the amnios.
(2444). Between the chorion (a) and the amnios (b) was an extensive vascular membrane (c, dd,ee): its figure seemed to have been that of a cone, of which the apex was at the umbilicus of the foetus.
(2445). Three vessels could be distinguished diverging from the umbilical cord, and ramifying over it. Two of these trunks contained coagulated blood; while the third was smaller, empty, and evidently the arterial trunk. No trace of any other membrane could be seen' extending from the foetus, besides the three above mentioned - the chorion (a), the amnios (6), and the interposed vascular membrane, the nature of which becomes the next subject of inquiry.
(2446). On tracing the three vessels above alluded to as ramifying over the vascular membrane through the umbilicus into the abdomen, the two larger ones, filled with coagulated blood, were found to unite, and, after being joined by the mesenteric vein, penetrated the liver: these, consequently, were the representatives of the onvphalo-mesenteric or vitelline vein of the embryo Bird (§ 2136.) The third vessel passed between the convolutions of the small intestine along the mesentery to the abdominal aorta, corresponding to an onvphalo-mesenteric or vitelline artery. The membrane, therefore, upon which they ramified answers to the vascular layer of the germinal membrane which spreads over the yelk in the oviparous animals, or to the vitelline vesicle of the embryo of ordinary Mammalia.
Fig. 417. Embryo of Kangaroo.
(2447). A filamentary pedicle connected this membrane to the intestine near the termination of the ilium, thus completing the resemblance between this apparatus and the vitelline system of Birds. But here we must caution the student not to be misled on one important point: the contents of the vitelline sac in the Marsupials, although doubtless intended to afford nourishment to the embryo animal, and thus representing the yelk of the Bird's egg, differ from it in one very essential circumstance. The yelk of the oviparous ovum is ready-formed in the ovary and exists prior to conception; but in the Mammal, where the ovarian yelk is met with in extremely small quantities, the contents of the vitellicle must obviously be derived from some other source, most probably from absorption from the uterine cavity.
(2448). In the Marsupial ovum the vascular membrane of the vitellicle is doubtless sufficient for the respiration of the little creature up to the time of its birth; and accordingly the allantoic system (§ 2139) is but very partially developed. In the ovum delineated in the last figure, there was as yet no perceptible trace either of an allantois or of a urinary bladder; but, as has been proved by another dissection, during the latter weeks of uterine gestation the urinary bladder is prolonged beyond the umbilicus so as to form a small allantois destined to receive the renal secretion, which becomes more abundant as the little foetus increases in size and completeness *.
(2449). In the mammary foetus of a Kangaroo a fortnight old, Professor Owen detected both a urachus and umbilical arteries; but these only extended from the bladder and iliac vessels as far as the umbilicus; neither could any umbilical vein be found penetrating the liver. It is in the placental Mammals that we shall find these vessels assuming their full importance, and developing themselves into a new system, whereby the communication between the mother and her offspring is still more effectually provided for.
(2450). When we consider the very early period at which the young Kangaroo is born, namely, at about the thirty-ninth day after conception, it is only reasonable to suppose that the organs most immediately connected with the vital actions are precociously matured; and accordingly, even in the embryo above delineated (fig. 417), the intestines, the liver, the kidneys, and the testes were all conspicuous, and the diaphragm, the heart, and the lungs were in such an advanced condition as to show that they would soon be capable of prematurely taking upon themselves the exercise of the circulatory and respiratory functions.
(2451). This rapid development of the viscera connected with circulation and respiration is, in truth, essentially requisite; for no sooner has the embryo arrived at the size represented in the next figure (fig. 418, a), the limbs being still in a most rudimentary condition, than the embryo is transferred from the uterus into the marsupial pouch, where it is found attached by its mouth to one of the nipples, from whence the materials of its support are to be obtained until it has acquired sufficient strength and size to leave the strange portable nest in which its foetal growth is accomplished and procure food adapted to a maturer condition.
* See Proceedings of the Zoological Society for August 1837.
(2452). A very beautiful provision is met with in the construction of the respiratory passages of the young Marsupial, intended to obviate the possibility of suffocation consequent upon the admission of milk into the trachea - a circumstance that, without some peculiar arrangement, might easily happen; but of this we must quote the original description, extracted from the paper already referred to *. "The new-born Kangaroo," observes Professor Owen, "possesses greater powers of action than the same-sized embryo of a Sheep, and approximates more nearly in this respect to the new-born young of the Rat; yet it is evidently inferior 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 1 and Mr. Morgan 2 have shown, is therefore provided with a 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 foetus.