THE FOZTAL MEMBRANES OF MAMMALS. 233 



yolk -sac and allantois, are also drawn out in the same manner into 

 two long tapering ends. 



On the entire surface of the chorion, with the exception of the 

 two ends of the sac, there have arisen rows of very vascular pads, 

 which radiate from separate smooth round spots of the membrane, 

 and are covered at their edges with small simple papillae. The 

 mucous membrane of the uterus is exactly fitted into the elevations 

 and depressions of the chorion. There are also found on it circular 

 smooth places similar to those of the chorion, which are further 

 noteworthy from the fact that it is only on them that the tubular 

 uterine glands open out. At birth the interlocking surfaces of 

 contact separate from each other without any loss of substance on 

 the part of the mucous membrane of the uterus ; for the pads and 

 small papillae are easily withdrawn from the depressions which serve 

 for their reception. 



In the third group a special organ, the placenta, or after- birth, 

 has been developed for the purpose of intra-uterine nutrition^ Its 

 origin was brought about by separate portions of the chorion having 

 assumed different characters, owing to the unequal size and distri- 

 bution of the villi. 



One part exhibits a condition in which the villi are entirely gone 

 or much stunted, so that the surface of the membrane feels smooth ; 

 moreover, it possesses few blood-vessels or is entirely destitute of them. 



Another part of the chorion contains, closely packed together, villi 

 which are extremely long and covered with many ramifying lateral 

 branches ; furthermore, it receives large blood-vessels, which approach 

 the tufts of villi and distribute their terminal capillaries t the finest 

 lateral ramifications of the latter; finally, it has entered into the 

 most intimate relations with the mucous membrane of the uterus. 

 Wherever the latter comes in contact with the tufts of villi it 

 is much thickened, very vascular, and in a state of active growth. 

 It encloses numerous branched cavities of varying size, into which 

 the villi of the chorion exactly fit. 



The entire structure is called a placenta, in which the part of the 

 chorion which is covered icith villi is distinguished as the placenta 

 fvetalis, and the part of the mucous membrane of the uterus which is 

 united with and adapted to the latter as the placenta uterina. Both 

 parts together constitute an organ for the nutrition of the embryo. 



The term placenta has often been extended to the kind of chorion 

 which is evenly covered with small villi, such as exists in the 

 Suidae, etc., and the designation of diffuse placenta has been created 



