202 WALTER HEAPE. 



The shape of the cells of this inner layer varies according 

 to age^ as van Beneden has observed (No. 4), but they invari- 

 ably have the aspect of an epithelial investment. To this 

 layer of cells the misleading term of membrana granulosa has 

 been applied. 



The Zona Radiata. 

 The zona radiata in fully ripe ova [vide figs. 1 and 2) is a 

 clear transparent membrane with a granular outer border upon 

 which the surrounding cells of the discus proligerus rest 



(fig.l)._ 



The inner portion of this membrane is so transparent 

 that the outlines of the epithelial cells may clearly be seen 

 through it. 



The origin of the granular outer portion has not been satis- 

 factorily traced; it may possibly, according to Balfour (No. 1), 

 be due to the presence of the remains of the primary vitelline 

 membrane, within which the zona radiata has been subse- 

 quently produced. On the other hand, the appearance may be 

 due to the irregularity of the surface of the zona radiata itself, - 

 this latter circumstance being in its turn occasioned partially 

 by the close adhesion of the surrounding cells of the discus 

 (fig. 6), partially by the open mouths of numerous canals 

 which pass radially through it, and to which I shall call atten- 

 tion directly (fig. 7). 



1 have not myself attempted in this paper to trace the 

 development of the ovarian ovum or its membranes, and must 

 therefore at present leave this question without further dis- 

 cussion. 



The thickness of the zona varies in the two specimens 

 represented (figs. 1 and 2) between "008 and -001 mm. The 

 two ova themselves were both completely surrounded by the 

 cells of the discus proligerus, but in the one drawn in fig. 2 

 the greater portion of these cells has been carefully detached. 



The radially striated appearance of the zona has long been 

 shown to be due to a vast number of fine canals passing i-adially 

 through it. These canals I find open on the inner side of the 



