MAMMALIA. 



255 



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). 



FIG. 159. SEMI-DIAGRAMMATIC VERTICAL SECTION THROUGH A PORTION OF A 



MATERNAL COTYLEDON OF A SHEEP. (From Turner.) 



cr. crypts; e. epithelial lining of crypts; v. veins and c. curling arteries of sub- 

 epithelial connective tissue. 



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 Lemuridas. 



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-deciduate 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 (a I}. The amnion (am} is enor- 

 mous, and is dorsally in apposition with, and apparently coalesces with 

 the chorion, and ventrally covers the inner wall of the persistent allantoic 

 sack. The chorion, except for a small area at the two poles and opposite 

 the os uteri, is nearly uniformly covered with villi, which are more nume- 



