MUSEUM OF COMPARATIVE ZOOLOGY. 65 



Tab. I. Fig. 8 B) the inner membrane is only slightly thicker than the 

 middle one, and both present a lighter appearance than the outer 

 one. 



I believe it is probable that the outer layer will be found to corre- 

 spond closely to the villous layer of the gar-pike. There can be no 

 doubt that the inner layer is the zona radiata, and I am inclined to 

 regard the middle layer simply as the differentiated outer half of the 

 zona j but the question can be answered satisfactorily only after renewed 

 investigations which give more particular attention to this point. The 

 principal reasons for my conclusion regarding the middle layer are, that 

 it evidently resembles the middle layer more than the outer, especially 

 in its capability of being stained, and that differences between the inner 

 and outer portions of the zona have been observed in the case of other 

 fishes. I know of no case, I admit, in which the outer half of the zona 

 may be easily removed with the villous layer, so that it still is possible 

 that the middle layer in Acipenser corresponds to the stalk region of the 

 villi in Lepidosteus. 



I do not clearly understand what the author means by saying that 

 the two membranes are derived from the two layers of cells which con- 

 stitute the epithelial wall of the follicle. It is true he claims that there 

 are two distinct cell layers, which, according to his figures of early 

 stages ('78 b , Tab. I. Figs. 5-7), are "granulosa" — next to the yolk — 

 and "follicular epithelium " — immediately outside the latter; but he 

 gives no figure showing both these at advanced stages of development. 

 I doubt their existence. But even if there were two separate epithelial 

 layers, I fail to understand how both could share in the production of 

 membranes which lie wholly inside the inner cell layer. 



The eggs of Acipenser are altogether unique so far as regards the con- 

 dition of the membrane in the micropylar region. 



The earliest observer of the micropyle was Kolliker ('57, p. 197), 

 who incidentally remarks that there is a single micropyle in the stur- 

 geon's egg ; but subsequent observers have claimed that there is a group 

 of several micropyles. 



Kowalevsky, Owsjannikow und Wagner (70 a , p. 172) state that 

 at one pole of the egg there are seven micropylar openings, — one in the 

 centre, with the other six arranged in a circle around the first. 



Salensky ('81, pp. 235, 236, Planche XV. Fig. 1 A) gives a more com- 

 plete account of the micropylar region, which is illustrated by figures. 

 He says : " At the germinative pole of the egg there is found a micropyle. 

 vol. xix. — no. 1. 5 



