364 



The Origin of the Zona Pellucida. 



Two opposing hypotheses have been advanced to account for the 

 origin of the zona pellucida. On the one hand Waldeyer (27 a) 

 supposed it to be the product of the activity of the cells of the 

 membrana granulosa, its outer granular layer coming directly from 

 these cells and the inner layer, the zona radiata, a modification of 

 this. While on the other hand van Beneden (4) maintained that it 

 was entirely of ovular origin. The latter author was led to this con- 

 clusion by the fact that in the polyovular follicles of the horse-shoe 

 bat that he examined the ova possessed a zona which was as well 

 developed on the sides of thera contiguous with other ova as on the 

 sides where it was covered by the membrana granulosa. Whereas 

 if it was a product of the follicular cells one would expect to find 

 it, at any rate thicker where it touched- these cells than in other 

 parts, if not indeed absent in the latter places. 



Honore (9) also inclines to this view for although in the fol- 

 licles he examined, the various ovules were always separated from 

 one another by membrana granulosa, in one instance he found an 

 ovum with a part of its surface not covered by these cells but free 

 in the liquor folliculi. The zona was of equal thickness throughout. 



The evidence from D. viverrinus also appears to favour this 

 hypothesis of van Beneden. In the first case the follicles were 

 always biovular with the two ova closely apposed for a considerable 

 part of their area. This was the case before the zona had begun, to 

 form as well as in all stages up to the ripe follicle and there was 

 no evidence to show that at any time the membrana granulosa had 

 penetrated between them. The same is true of the second example, 

 but here owing to the presence of more than two ovules in a follicle 

 a larger part of the superficial area of each egg was out of contact 

 with the walls of the follicle. 



The origin of Polyovular Follicles. 

 Two views have also been put forward with regard to the origin 

 of polyovular follicles. Schottländer (19) suggests that they may 

 be the result of the division of one primordial ovum or, again, they 

 may be two separate primordial ova which have been accidentally 

 brought together by the conjunctive tissue into a single follicle. 

 For various reasons the author inclmes to the latter view. Stoeckel (23), 

 however, maintains that the various ova in the one follicle have arisen 



