MAMMALIA. 



253 



chorionic ridges are highly vascular (fig. 155). On the other hand, there are 

 non-vascular circular depressions corresponding to the non-vascular areas 

 on the chorion ; and in these areas, and in these alone, the glands of the 

 uterus open (fig. 155 g) (Turner). The maternal and fcetal parts of the 

 placenta in the pig separate with very great ease. 



FIG. 156. VERTICAL SECTION THROUGH THE INJECTED PLACENTA OF A MARE. 



(From Turner.) 



ch. chorion with its villi partly in situ and partly drawn out of the crypts (cr); 

 E. loose epithelial cells which formed the lining of the crypt; g. uterine glands; 

 v. blood-vessels. 



In the mare (Turner), the fcetal villi are arranged in a less definite 

 zonary band than in the pig, though still absent for a very small area at 

 both poles of the chorion, and also opposite the os uteri. The filiform villi, 

 though to the naked eye uniformly scattered, are, when magnified, found to 

 be clustered together in minute cotyledons, which fit into corresponding 

 uterine crypts (fig. 156). Surrounding the uterine crypts are reticulate 

 ridges on which are placed the openings of the uterine glands. The re- 

 maining Ungulata with diffused placentae do not differ in any important 

 particulars from those already described. 



The polycotyledonary form of placenta is found in the Ruminantia 

 alone. Its essential character consists in the fcetal villi not being uni- 

 formly distributed, but collected into patches or cotyledons which form as 

 it were so many small placentae (fig. 157)- The fcetal villi of these patches 

 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 structures is so close that large parts of the maternal 



