MA MM ALT A. 



729 



the functions both of the vitellicle and of the allantoic apparatus 

 become no longer efficient either for nutrition or respiration, a 

 third system of organs is developed in the placental Mammifer, 

 whereby a vascular intercommunication is established between the 

 fetus and the uterine vessels of the mother, forming what has been 

 named by human embryologists the Placenta. 



In the ovum of a Sheep, at that period of the growth of the fetus 

 which nearly corresponds with the end of utero-gestation in the 

 prematurely born Kangaroo, all the three systems alluded to are 

 coexistent and easily distinguishable, as will be seen in the accom- 

 panying figure (j&\ 336). The fetus (0), enclosed in its amnio- 

 tic membrane (7>), has its limbs as yet but very imperfectly formed, 

 exhibiting pretty nearly the condition of a nascent Marsupial (vide 

 Jig. 331) ; but here it will be seen that the umbilical systems exhi- 

 bit very striking differences in the two races. The vitellicle (f), 



Fig. 336. 



with its pedicle (e), are of very small dimensions ; the allantoid 

 sac (g), on the contrary, is of considerable bulk, and, having ceased 

 to act as a respiratory organ, becomes adapted to receive the urinary 

 secretion through the canal of the urachus. The most important 

 feature, however, is the" rapid extension of the umbilical vessels (rf), 

 which in BIRDS and MARSUPIALS were distributed only to the 

 allantois ; but in the placental Mammals these vessels rapidly spread 

 over the chorion (A), and, coming in contact with the vascular sur- 

 face of the womb, they soon form a new bond of communication be- 

 tween the mother and the fetus, constituting the placenta ; and thus 

 the offspring is nourished, until, its intra-uterine growth being ac- 

 complished, it is born in an advanced condition of developement, 

 and becomes the object of maternal care during that period in 

 which it is dependent upon the breast of its mother for support. 



