60 J. T. CUNNINGHAM. 



dered ; we can obtaiu some probable conclusions concerning it 

 by referring to what is known concerning the egg membranes 

 in other Vertebrata. The account which Balfour gives in his 

 ' Corap. Emb./ vol. i, of the egg membranes in Craniata, is as 

 follows. There are three membranes which may all coexist, 

 or one or two only may be present. These are : 



1. An outermost homogeneous membrane without striae or 

 fine pores, by most authors regarded as a chorion (i. e. as pro- 

 duced by the follicular epithelium), but by Balfour as a vitelline 

 membrane (i. e. as produced by the ovum itself). 



2. A radiately striated membrane, internal to the former 

 when the two coexist, which can be broken up into a series of 

 separate columns. These give to the membrane its radiate 

 striation, but it is probable that there are fine pores between 

 the columns. This membrane is the zona radiata of most 

 authors. It is a differentiation of the outermost layer of 

 the yolk. 



3. Within the zona radiata a third and delicate membrane 

 is occasionally found, especially when the ovum is approaching 

 maturity. 



According to Balfour, the first membrane to be formed in 

 Elasmobranchs is the vitelline membrane, the first of the three 

 above defined ; this appears in some instances before the forma- 

 tion of the follicle, a fact which appears to show that it is 

 really formed as a differentiation of the protoplasm of the eg^. 

 In Elasmobranchs this membrane attains a very considerable 

 development. A zona radiata is generally if not always present 

 in Elasmobranchii, but arises later than the vitelline mem- 

 brane. The zona radiata always disappears long before the 

 ovum is ripe. The vitelline membrane also gradually atrophies 

 though it lasts much longer than the zona radiata. When the 

 egg is taken up by the oviduct, all trace of both membranes 

 has disappeared. 



Is there any evidence to show whether the membrane (c) in 

 the Myxine ovum owes its existence to the ovum itself or to the 

 follicular epithelium ? The only evidence to which I will point 

 at present is that the follicular epithelium is very much thicker 



