REPRODUCTION, 89 
vessels are said to constitute large intercommunicating sinuses ; 
the villi may hang freely suspended in these sinuses, or be 
anchored to their walls by strands of tissue. There is believed 
to be only one layer of epithelial cells between the vessels of 
mother and fcetus in the later stages of pregnancy. This, 
while closely investing the foetal vessels (capillaries), really 
belongs to the maternal structures. The significance 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 remem- 
bered that it is at one period represented by the chorion with 
its villi distributed universally. 
The resemblance in 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 complications 
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 
