21-2 



DIFFUSED PL A CENT A . 



fit into corresponding pits in thickened patches of the wall of the uterus 

 (figs. 158 and 159). In many cases (Turner), the interlocking of the 

 maternal and fcetal structiires is so close that large parts of the maternal 

 epithelium are carried away when the foetal villi are separated from the 



FIG. 159. SEUI-DIAGKAMMATIC VERTICAL SECTION THROUGH A PORTION OF A MATERNAL 



COTYLEDON OF A SHEEP. (From Turner.) 



cr. crypts; e. epithelial lining of crypts; r. veins and c. curling arteries of suli- 

 epithelial connective tissue. 



uterus. The glands of the uterus open in the intervals between the 

 cotyledons. The character of the cotyledons differs greatly in different 

 types. The maternal parts are cup-shaped in the sheep, and mushroom- 

 shaped in the cow. There are from 60 100 in the cow and sheep, but 

 only about five or six in the Roe-deer. In the Giraffe there are, in addition 

 to larger and smaller cotyledons, rows and clusters of short villi, so that the 

 placenta is more or less intermediate between the polycotyledonary and 

 diffused types (Turner). A similarly intermediate type of placenta is found 

 in Cervus mexicanus (Turner). 



The groups not belonging to the Ungulata which are characterized by 

 the possession of a diffused placenta are the Sirenia, the Cetacea, Manis, 

 and the Lemuridse. 



Sirenia. Of the Sirenia, the placentation of the Dugong is known 

 from some observations of Harting (No. 201). 



It is provided with a diffuse and non-decichiate placenta ; with the 

 villi generally scattered except at the poles. The umbilical vesicle vanishes 

 early. 



Cetacea. In the Cetacea, if we may generalize from Turner's observa- 

 tions on Orca Gladiator and the Narwhal, and those of Anderson (No. 191) 

 on Platanista and Orcella, the blastodermic vesicle is very much elongated, 

 and prolonged unsymmetrically into two horns. The mesoblast (fig. 160) 

 of the allantois would appear to grow round the whole inner surface of the 

 subzonal membrane, but the cavity of the allantois only persists as a widish 

 sack on the ventral aspect of the embryo (al). The amnion (am) is enor- 



