MUSEUM OF COMPARATIVE ZOOLOGY. 51 



be inferred that the zona radiata is exclusively the product of the yolk. It 

 is also probable, from the evidence of stratification sometimes seen in 

 the completed structure, that the zona is produced in successive layers. 

 If such is the case, it follows that portions of the zona nearer the yolk 

 are formed after those which have a more peripheral position. 



The question as to the source of the villous layer is not so easily an- 

 swered. The fundamental difference between it and the zona radiata at 

 once suggests for it a different origin. If the latter arises from the yolk, 

 the former might be produced by the follicular epithelium. This view 

 would seem to receive confirmation from the peculiar way in which the 

 roots of the villi in the mature egg penetrate the pore-canals of the zona 

 radiata. I have no doubt that this condition would be regarded by 

 many observers as a welcome confirmation of the theory that the pore- 

 canals are primarily for the purpose of transmitting nutritive material 

 to the growing egg. Such observers might look upon the villi as secre- 

 tions from the granulosa, which, owing to slight physical and chemical 

 changes, had not passed through the pore-canals as nutriment, but re- 

 mained partly outside the zona to subserve other functions. This view 

 might be further supported by the fact that during the formation of the 

 villi the inner surfaces of the granulosa cells are not sharply marked off 

 by membranes from the underlying structures. 



Nevertheless, it seems to me that the arguments which may be 

 adduced to support the opposite view, — that the villous layer is the 

 product of the secretive activity of the ovum itself, — greatly outweigh 

 these considerations. 



During the early stages of their formation the villi are so intimately 

 related to the ovum that they appear to be rods imbedded in its sub- 

 stance, and at no time during its formation is the villous layer separable 

 from the yolk. If the latter is by any means removed from the mem- 

 brane, there is always a superficial portion of the ovum which remains 

 attached to its inner surface. The separation of the granulosa cells 

 from the membrane during this period, on the contrary, is quite com- 

 mon. What might otherwise be a 'serious obstacle to considering the 

 villi the product of the ovum, — the presence of a zona between the 

 two, — is entirely nullified by the fact, previously established, that the 

 villous layer is produced before the zona radiata. 



Whatever renders improbable the formation of the villi from the fol- 

 licular epithelium is, of course, favorable to the opposite view. If the 

 villi were products of the epithelium, one would expect some constancy 

 in the numerical relations between the two, but this is certainly wanting. 



