MUSEUM OF COMPARATIVE ZOOLOGY. 63 



The size of the ova at the time of the maximum development of the 

 membranes is not stated ; but after this stage is reached both mem- 

 branes gradually atrophy. " The zona is first to disappear, and the 

 vitelline membrane next becomes gradually thinner. Finally, when the 

 egg is nearly ripe, the follicular epithelium is separated from the yolk 

 by an immeasurably thin membrane, — the remnant of the vitelline 

 membrane," which is no longer visible when the egg becomes detached 

 from the ovary. Both vitelline membrane and zona are found in Raja, 

 but in a much less developed condition than in Scyllium, and the zona 

 is developed at a much later period than in that species. 



If the account given by Balfour is correct, — and his description 

 seems to be both more complete and more accurate than that of 

 Schultz, — then there is an interesting parallelism between the pri- 

 mary egg membranes in selachians and those of Lepidosteus and the 

 bony fishes. Not that the villous layer in Lepidosteus is structurally 

 comparable with that which Balfour calls in sharks a vitelline mem- 

 brane, but genetically they are alike. They are the membranes which are 

 first to be produced, — i. e. before the zona radiata, — and they are in 

 both instances the product of the ovum, not of the follicular epithelium. I 

 shall take occasion later to refer to the theoretical importance of this 

 discovery by Balfour. 



The ultimate disappearance of both membranes renders the formation 

 of a micropyle superfluous. It would be interesting to learn, however, 

 whether there is at any time a trace of such a structure. 



c. Ganoidei. 



The only ganoids besides Lepidosteus whose egg membranes have 

 been described are Amia and Acipenser. I have elsewhere (p. 27) 

 quoted Ryder's account of the membranes in the case of Amia. 



From the accounts given by Kolliker, by Kowalevsky, Owsjannikow 

 und Wagner, and by Salensky, I believe there must be considerable 

 similarity between the conditions of the egg membranes in Acipenser 

 and Lepidosteus. 



Kolliker ('57, p. 197) says that the porous membranes in the case 

 of the sturgeon form "three layers; two inner, darker, thinner, closely 

 porous, and an outer pale, thicker layer, apparently with fewer pores. 

 This outer layer, which is also softer than the others, shows its outer 

 surface divided into small polygonal areas, which appear to correspond 

 to the epithelial cells of the egg capsule [follicle]. These cells are ex- 

 tremely delicate and pale, but yet seen from the surface they show a 



