REPRODUCTIOK. 93 



cance of this general arrangement will be explained in the 

 chapter on the physiological aspects of the subject. 



It remains to inquire into the relation of these forms to one 

 another from a phylogenetic (derivative) point of view, or to 

 trace the evolution of the placenta. 



Evolution. — Passing by the lowest mammals, in which the 

 placental relations are as yet imperfectly understood, it seems 

 clear that the simplest condition is found in the rodentia. 

 Thus, in the rabbit, as has been described, both yelk-sac and 

 allantois take a nutritive part ; but the latter remains small. 

 In forms above the rodents, the allantois assumes more and 

 more importance, becomes larger, and sooner or later predomi- 

 nates over the yelk-sac. 



The discoidal, zonary, cotyledonary, etc., are plainly evolu- 

 tions from the diffuse, for both differentiation of structure and 

 integration of parts are evident. The human placenta seems 

 to have arisen from the diffuse form ; and it will be remembered 

 that it is at one period represented by the chorion with its villi 

 distributed universally. 



The resemblance of the embryonic membranes at any early 

 stage in man and other mammals to those of birds certainly 

 suggests an evolution of some kind, though exactly along what 

 lines that has taken place it is difficult to determine with exact- 

 ness ; however, as before remarked, nearly all the complica- 

 tions of the higher forms arise by concentration and fusion, on 

 the one hand, and atrophy and disappearance of parts once 

 functionally active, on the other. 



Summary. — The ovum is a typical cell ; unspecialized in most 

 directions, but so specialized as to evolve from itself compli- 

 cated structures of higher character. The segmentation of the 

 ovum is usually preceded by fertilization, or the union of the 

 nuclei of male and female cells, which is again preceded by the 

 extrusion of polar globules. In the early changes of the ovum, 

 including segmentation, periods of rest and activity alternate. 

 The method of segmentation has relation to the quantity and 

 arrangement of the food-yelk. , Ova are divisible generally 

 into completely segmenting (holoblastic), and those that under- 

 go segmentation of only a part of their substance (meroblastic) ; 

 but the processes are fundamentally the same. 



Provision is made for the nutrition, etc., of the ovum, when 

 fertilized (oosperm) by the formation of yelk-sac and allan- 

 tois; as development proceeds, one becomes more prominent 



