Development of.the Mammal 309 



is attached to the foetus at the umbilicus, while 

 distally it is continuous with the foetal placenta. 

 Its structure, as seen in cross section, varies 

 with the period of pregnancy and somewhat 

 with the plane of the section. 



Figure 100 shows two sections of the cord, 

 at different periods. In the younger section, 

 which is, of course, the smaller, there is a con- 

 siderable portion of the body-cavity ; the yolk- 

 stalk and allantois are well marked, while the 

 two arteries and one vein (formed by the fusion 

 of the two original veins) are comparatively 

 small. In the older section the body-cavity is 

 smaller or absent, the yolk-stalk and allantois 

 are less distinct, or even invisible, and the blood 

 vessels are larger. The greater part of the 

 substance of the cord is made up of angular or 

 stellate rnesoblast cells which form a sort of 

 reticulum, the meshes of which are filled with 

 fibres and a soft, jelly-like substance ; it is often 

 called jelly of Wharton. Surrounding the 

 jelly of Wharton is a boundary of ectoderm 

 consisting of three or four layers of cells. 



The human placenta, as has been said, con- 

 sists of two parts, the maternal and the foetal ; 

 the former is simply the much-thickened decidua 

 serotina, in whose villi very numerous blood 



