68 J. T. CUNNINGHAM. 



substance of the cells being partly transformed into the sub- 

 stance of the membrane, while threads of protoplasm, at more 

 or less regular intervals, remain unchanged, and thus give rise 

 to the pores of the membrane. Od this hypothesis — for it is 

 little more — any membrane which may be formed by the ovum 

 itself is to be sought on the inner side of the zona radiata. 

 In Myxine I have found no such membrane. I have not 

 hitherto had an opportunity of comparing the structure of the 

 egg follicle in Petromyzon with that in Myxine. According 

 to Calberla the ovum in Petromyzon is surrounded by a zona 

 radiata in two layers, both radiately striated. According to 

 KupfFer and Benecke the outer layer is not striated. Calberla 

 asserts the existence of a micropyle, while Kupffer and Benecke 

 have thrown doubts on the presence of such an aperture. 

 Judging from what I have seen in Myxine I am inclined to 

 think that Calberla is correct, and that the condition of things 

 in Petromyzon is exactly similar to that in Myxine, except 

 that the zona radiata in the former is, as in Perca, divided 

 into two layers. In the ovum of Myxine I have observed in 

 some sections a very faint trace of a separation into two layers, 

 and have indicated this in fig. 12, but I do not think this (in 

 the absence of any difference in character in the two layers, 

 and in view of the fact that the line of separation can never 

 be traced continuously for any distance) indicates difference of 

 origin of the two strata of the membrane. The membrane is 

 not developed till a late stage in the growth of the ovum. 



In fig. 13 is shown a section through the pole of an ovarian 

 ovum 11 mm. in length. The germinal disc is seen to have the 

 same characters as it has in the more mature ova, except that 

 the germinal vesicle is elliptical in section. This latter pecu- 

 liarity is probably an accident caused by the action of reagents. 

 Under a system magnifying 50 diameters, the follicular epithe- 

 lium at this stage can scarcely be distinguished, while the con- 

 nective-tissue layer is very thick. On applying a higher 

 magnifying power it is seen that no membrane of any kind 

 exists at this stage between the follicular epithelium and the 

 vitellus. The epithelium itself is only one cell thick, and the 



