716 MAMMALIA. 



from the fetus besides the three above mentioned, the chorion 

 (a), the amnios (i), and the interposed vascular membrane, the 

 nature of which becomes the next subject of inquiry. 



(843.) On tracing the three vessels above alluded to, as ramify- 

 ing 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 pene- 

 trated the liver : these, consequently, were the representatives of 

 the omphalo-mesenteric or vitelline vein of the embryo bird 

 ( 703). The third vessel passed between the convolutions of the 

 small intestine along the mesentery to the abdominal aorta, corre- 

 sponding to an omphalo-mesenteric or vitelline artery. The mem- 

 brane, therefore, upon which they ramified answers to the vascular 

 layer of the germinal membrane which spreads over the yolk in 

 the Oviparous animals, or to the vitelline vesicle of the embryo of 

 ordinary Mammalia. 



A filamentary pedicle connected this membrane to the intestine 

 near the termination of the ileum, thus completing the resem- 

 blance between this apparatus and the vitelline system of Birds. 

 But here we must caution the student not to be misled on one im- 

 portant point : the contents of the vitelline sac in the Marsupials, 

 although doubtless intended to afford nourishment to the embryo 

 animal, and thus representing the yolk of the bird's egg, differs 

 from it in one very essential circumstance. The yolk of the Ovi- 

 parous ovum is ready formed in the ovary and exists prior to con- 

 ception ; but in the Mammal, where the ovarian yolk 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. 



(844). 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 allan- 

 toic system ( 705) 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 week of uterine ges- 

 tation, 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 fetus increases in size 

 and completeness.* 



* See Proceedings of the Zool. Society for August, 1837. 



