62 BULLETIN OF THE 



homogeneous layer persist unchanged up to the bursting of the follicle. 

 It is from this part that those granulosa cells come which are occasion- 

 ally encountered on the escaped [^egg - ] and within the empty follicle." 



On eggs of Acanthias, Scymnus, and Mustelus, Schultz found a 

 homogeneous layer joined with the follicular layer, and inside the latter 

 a zona radiata, the inner margin of which was sharply defined against 

 the yolk. " The pores of this cuticular zona were traversed by proto- 

 plasmic processes, which stretched from the homogeneous layer to the 

 egg protoplasm and fused with the latter." 



The author concludes that, so far as his own observations reach, there 

 are to be distinguished in selachians the four "following conditions 

 of the follicular epithelium " : (a) simple epithelium (embryonic stage 

 of selachians) ; (6) epithelium with homogeneous basal margin (Tor- 

 pedo) ; (c) epithelium with homogeneous perforate basal margin (Raja) ; 

 (d) epithelium with broad homogeneous and narrower perforate basal 

 margin (Squalidse). 



Balfour ('78 b , pp. 402, 403) has confirmed the existence and subse- 

 quent disappearance of two membranes — an outer homogeneous, and 

 an inner striate — in one of the Squalidse, Scyllium ; but he believes 

 that they are produced by the ovum, not by the follicular epithelium, 

 and that they are absorbed, not converted into connective tissue. Two 

 similar membranes are also found in Raja, and are believed by Balfour 

 to be common probably to all sharks. The homogeneous membrane is 

 formed before the striate one. In Scyllium " the [homogeneous] mem- 

 brane would seem indeed to be formed in some instances even before the 

 ovum has a definite investment of follicular cells. ,, Consequently it is 

 called a vitelline membrane. In ova 0.12 mm. in diameter it is not thick 

 enough to be accurately measured ; in those of 0.5 mm. diameter it has 

 a thickness of 2 ll, and there may also be observed inside it faint indi- 

 cations of the differentiation of the outermost layer of the vitellus into 

 the perforate or radially striate membrane of Schultz. The latter Bal- 

 four does not hesitate to call a zona radiata. 



In ova 1 mm. in diameter the zona has increased in thickness (to 4 fx) 

 and " is always very sharply separated from the vitelline membrane, but 

 appears to be more or less continuous on its inner border with the body 

 of the ovum, at the expense of which it no doubt grows in thickness." 

 In larger eggs both membranes increase in thickness, especially the zona, 

 which now becomes marked off from the yolk. " In many specimens it 

 appears to be formed of a number of small columns as described by 

 Gegenbaur [for the alligator] and others." 



