94 INTRODUCTION TO CLASSIFICATION. 



texture and traversed by large venous channels in its inner sub- 

 stance, c ; d is a decidual layer of the uterus of a cavernous 

 structure, whence vascular processes are continued towards the 

 chorionic surface of the placenta. A large vein (*') passes 

 directly from the decidual layer (d), and the uterine sinuses 

 beneath it, to near the chorionic surface of the placenta, beneath 

 which it branches out horizontally. The chorion (/), rendered 

 vascular over its non-placental part by the omphalo-meseraic 



Fig. 43. 



Fig. 43. Magnified view of a section of the placenta and uterus of a pregnant Eat. 



vessels (/<;) only begins to exhibit villous processes and folds at 

 the point (g). These outermost villi appear to me to be free ; 

 but, more internally, they become closely connected with the 

 upper surface of the placenta ; and over the central third of the 

 foetal face of the placenta, the umbilical vessels (?) ramify in a 

 radiating fashion, and send prolongations down between the deci- 

 dual lamellae. The slightest traction exerted upon the cord 

 causes the placenta to separate along the line e, m, m, e, bringing 

 with it, of course, the cup-shaped decidua, d. 



The Carnivora develop a well-marked decidua, but their 

 placenta in all genera which have been examined (except the 

 Polecat, according to Von Baer) has the form of a complete 

 zone, or broad girdle, surrounding the middle of the chorion 

 and leaving the poles bare (Fig. 44). 



Thus Man ; the Apes, or so-called Quadrumana ; the 



