FCETUS IN UTEKO. 



839 



intercommunication is established between the fo3tus and the uterine 

 vessels of the mother, forming what has been named by human embryo- 

 legists the placenta. 



(2488.) In the ovum of a Sheep, at that period of the growth of the 

 foetus which nearly corresponds with the end of uterogestation in the 

 prematurely-born Kangaroo, all the three systems alluded to are co- 

 existent and easily distinguishable, as will be seen in the accompanying 

 figure (fig. 423). The foetus (a), enclosed in its amniotic membrane (6), 

 has its limbs as yet but very imperfectly formed, exhibiting pretty- 

 nearly the condition of a nascent Marsupial (vide fig. 418) ; but here it 

 will be seen that the umbilical systems exhibit very striking differences 

 in the two races. The vitdlide (/), with its pedicle (e) } are of very 

 small dimensions ; the allantoid sac (g), on the contrary, is of con- 

 siderable bulk, and, having ceased to act as a respiratory organ, becomes 

 adapted to receive the urinary secretion through the canal of the 



Fig. 423. 



Embryo of the Sheep. 



urachus. The most important feature, however, is the rapid extension 

 of the umbilical vessels (d), which in Birds and Marsupials were dis- 

 tributed only to the attantois, but in the placental Mammals these 

 vessels rapidly spread over the chorion (Ji), and, coming in contact with 

 the vascular surface of the womb, they soon form a new bond of com- 

 munication between the mother and the foetus, constituting the placenta; 

 and thus the offspring is nourished until, its intra-uterine growth 

 being accomplished, it is born in an advanced condition of development, 

 and becomes the object of maternal care during that period in which it 

 is dependent upon the breast of its mother for support. 



(2489.) The appearance of the placenta varies much in different 

 tribes : thus in the Sheep and other Ruminants it consists of numerous 

 detached masses of villi (fig. 423, i i\ that interdigitate with correspond- 

 ing processes derived from the maternal womb ; in the Mare it covers the 

 whole surface of the chorion ; but in the greater number of Mammals, 



