558 



EMBRYOLOGY. 



scattered evenly over the chorion, so that the placenta is diffuse " 

 (Huxley). In carnivora the villi are collected in a mass which 

 assumes the form of a belt, surrounding the chorion, and hence 

 called a zonary placenta. In rodentia the placenta is discoid, 

 the villi being assembled in a more or less circular mass. 



The placenta is found in most orders of mammals, and in them 

 alone ; its presence or absence has led to the separation of the 

 class mammalia into a placental and an implacental division. 



FIG. 216. 



Foetus of the Cow, with its membranes, a a, Placenta ; 6 b, Chorion with the Allantois adherent to 

 its inner surface ; c c, Amnion seen through outer covering ; d, Foetus seen through its coverings. 



UMBILICAL CORD. 



This structure is the medium of connection between the foetus 

 and the placenta, reaching from the latter to the umbilical open- 

 ing in the linea alba. It consists mainly of the umbilical vessels, 

 one vein, and two arteries, and superiorly contains the urachus, 

 a foetal tube continuous with the bladder. 



