828 MAMMALIA. 



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. 



(2447.) A filamentary pedicle connected this membrane to the in- 

 testine near the termination of the ilium, 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 important 

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

 doubtless intended to aiford 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 vitel- 

 licle 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 pro- 

 longed 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, Pro- 

 fessor 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 concep- 

 tion, it is only reasonable to suppose that the organs most immediately 

 connected with the vital actions are precociously matured ; and accord- 

 ingly, 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 circu- 

 lation and respiration is, in truth, essentially requisite ; for no sooner 

 * See Proceedings of the Zoological Society for August 1837. 



