OF THE VERTEBRATE OVARY. 605 



the stroma cells when viewed (as is ordinarily the case) in optical 

 section. 



In the innermost layer many of the follicles with the enclosed 

 ova have advanced considerably in development and are formed 

 of columnar cells. The somewhat heterodox view of these cells 

 propounded by Foulis I cannot quite agree to. He says (Quart. 

 y. Mic. Set., Vol. XVI., p. 210): "The protoplasm which sur- 

 rounds the vesicular nuclei acts as a sort of cement substance, 

 holding them together in the form of a capsular membrane 

 round the young ovum. This capsular membrane is the first 

 appearance of the membrana granulosa." I must admit that I 

 find nothing similar to this, nor have I met with any special 

 peculiarities (as Foulis would seem to indicate) in the cells of the 

 germinal epithelium or other cells of the ovary. 



Figure 41 is a representation of an advanced follicle of a six 

 weeks' rabbit, containing two ova, which is obviously in the act 

 of dividing into two. Follicles of this kind with more than one 

 ovum are not very uncommon. It appears to me probable that 

 follicles, such as that I have figured, were originally formed of 

 a single mass of protoplasm with two nuclei ; but that instead 

 of one of the nuclei atrophying, both of them eventually de- 

 veloped and the protoplasm subsequently divided into two 

 masses. In other cases it is quite possible that follicles with 

 two ova should rather be regarded as two follicles not separated 

 by a septum of stroma. 



On the later stages of development of the ovary I have no 

 complete series of observations. The yolk spherules I find to 

 be first developed in a peripheral layer of the vitellus. I have 

 not been able definitely to decide the relation of the zona radiata 

 to the first formed vitelline membrane. Externally to the zona 

 radiata there may generally be observed a somewhat granular 

 structure, against which the follicle cells abut, and I cannot 

 agree with Waldeyer (loc cit., p. 40) that this structure is con- 

 tinuous with the cells of the discus, or with the zona radiata. 

 Is it the remains of the first formed vitelline membrane ? I have 

 obtained some evidence in favour of this view, but have not been 

 successful in making observations to satisfy me on the point, 

 and must leave open the question whether my vitelline mem- 

 brane becomes the zona radiata or whether the zona is not a 



