MUSEUM OF COMPARATIVE ZOOLOGY. 53 



At first thought one might regard the modifications of the villous layer 

 in the micropylar region as the direct result of an alteration in the secre- 

 tive powers of the granulosa cells situated at that place ; but it seems 

 to me that the thickness of the layer ought, on this assumption, to be 

 greater than elsewhere, since the granulosa cells are here more numerous 

 and larger. Besides, the corresponding diminution in the thickness of 

 zona radiata could not be thus accounted for, but must be assumed to 

 be the result of diminished secreting activity on the part of the ovum in 

 this region. Hence the same explanation would certainly be more rea- 

 sonable in the case of the villous layer. This is a point which seems to 

 me of considerable importance; the diminished activity of this region 

 which is shown during the formation of the zona was already manifest 

 during the formation of the villi. 1 



From these several considerations, I believe there can be little ques- 

 tion that the villous layer of the egg membranes in Lepidosteus is also 

 the product of the ovum itself rather than of the follicular epithelium 

 surrounding it. 



If this conclusion is established, it follows that the parts of the villi 

 first to be produced are those which are most superficial. I believe that 

 this is confirmed by the fact that the forming villi are readily stained in 

 carmine. It is probable that, even in the latest stage of the immature 

 eggs (0.7 mm.) which I have seen, not much, if anything, more than the 

 heads of the villi have been produced. The length and the highly re- 

 fractive condition of the villi at this stage, and the fact that they are 

 not at all folded, all point to this conclusion. 



There still remains much to be done in following out the exact course 

 of the development of the membranes in Lepidosteus, — especially in 

 determining when the formation of the zona begins in relation to the 

 completion of the villous layer, — but I think that the main features of 

 the process as outlined above will not be disproved by subsequent 

 study. 



I have no explanation to offer of the apparently sudden change in the 

 nature of the secretions from the ovum which is registered in the pro- 

 duction of structures so dissimilar as the zona and villous layer are ; 

 but it is possible that some light may be thrown on this question when 

 the period of the transition has been carefully worked out. 



1 This is an evidence of the polar differentiation of ova (which exhibits itself in 

 many other phenomena) to which attention has not hitherto been called. 



