158 WALTER HEAPE. 
The shape of the cells of this inner layer varies according 
to age, as van Beneden has observed (No. 4:), but they invariably 
have the aspect of an epithelial investment. To this layer of 
cells the misleading term of membrana granulosa has been 
applied. 
Tue Zona Raprata. 
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 
(ig); 
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 attention 
directly (fig. 7)). 
I 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 
discussion. 
The thickness of the zona varies in the two specimens 
represented (figs. 1] and 2) between ‘008 and ‘011 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 radially 
through it. These canals I find open on the inner side of the 
