OF THE VERTEBRATE OVARY. 607 



(4) The conversion of the cells of the germinal epithelium 

 into permanent ova commences in an embryo of about twenty- 

 two days. All the cells of the germinal epithelium appear to 

 be capable of becoming ova : the following are the stages in 

 the process, which are almost identical with those in Elasmo- 

 branchs : 



(a) The nucleus of the cells loses its more or less distinct 

 network, and becomes very granular, with a few specially large 

 granules (nucleoli). The protoplasm around it becomes clear 

 and abundant primitive ovum stage. It may be noted that 

 the largest primitive ova are very often situated in the pseudo- 

 epithelium, (b) A segregation takes place in the contents of 

 the nucleus within the membrane, and the granular contents 

 pass to one side, where they form an irregular mass, while the 

 remaining space within the membrane is perfectly clear. The 

 granular mass gradually develops itself into a beautiful reticu- 

 lum, with two or three highly refracting nucleoli, one of which 

 eventually becomes the largest and forms the germinal spot par 

 excellence. At the same time the body of the ovum becomes 

 slightly granular. While the above changes, more especially 

 those in the nucleus, have been taking place, the protoplasm of 

 two or more ova may fuse together, and polynuclear masses be 

 so formed. In some cases the whole of such a polynuclear mass 

 gives rise to only a single ovum, owing to the atrophy of all the 

 nuclei but one, in others it gives rise by subsequent division to 

 two or more ova, each with a single germinal vesicle. 



(5) All the cells of a nest do not undergo the above changes, 

 but some of them become smaller (by division) than the indif- 

 ferent cells of the germinal epithelium, arrange themselves round 

 the ova, and form the follicular epithelium. 



(6) The first membrane formed round the ovum arises in 

 some cases even before the appearance of the follicular epithe- 

 lium, and is of the nature of a vitelline membrane. It seems 

 probable, although not definitely established by observation, 

 that the zona radiata is formed internally to the vitelline mem- 

 brane, and that the latter remains as a membrane, somewhat 

 irregular on its outer border, against which the ends of the follicle 

 cells abut. 



