STRUCTURE AND DEVELOPMENT OF VERTEBRATE OVARY 403 



The zona becomes marked off from the yolk, and its radial striae 

 become easy to see even with comparatively low powers. In 

 many specimens it appears to be formed of a number of small 

 columns, as described by Gegenbaur and others. The stage of 

 about the greatest development of both the vitelline membrane 

 and zona radiata is represented on PI. XVIII, Fig. 23. 



At this time the vitelline membrane appears frequently to 

 exhibit a distinct stratification dividing it into two or more suc- 

 cessive layers. It is not, however, acted on in the same manner 

 by all reagents, aiid with absolute alcohol appears at times longi- 

 tudinally striated. 



From this stage onwards, both vitelline membrane and zona 

 gradually atrophy, simultaneously with a series of remarkable 

 changes which take place in the follicular epithelium. The zona 

 is the first to disappear, and the vitelline membrane next 

 becomes gradually thinner. Finally, when the egg is nearly 

 ripe, the follicular epithelium is separated from the yolk by an 

 immeasurably thin membrane — the remnant of the vitelline 

 membrane — only visible in the most favorable sections 

 (PI. XVIII, fig. 23 V t.). When the egg becomes detached 

 from the ovary even this membrane is no longer to be 

 seen. 



Both the vitelline membrane and the zona radiata are found in 

 Raja, but in a mucli less developed condition than in Scyllium. 

 The vitelline membrane is for a long time the only membrane 

 present, but is never very thick (PI. XIX, fig, 31). The zona is 

 not formed till a relatively much later period than in Scyllium, and 

 is always delicate and difiicult to see (PI. XIX, fig. 32). Both 

 membranes atrophy before the egg is quite ripe ; and an ap- 

 ])arently fluid layer between the follicular epithelium and the 

 vitellus, which coagulates in hardened specimens, is probably 

 the last remnant of the vitelline membrane. It is, however, 

 much thicker than the corresponding remnant in Scyllium. 



Though I find the same membranes in Scyllium as Alexander 

 Schultz did in other Squalidse, my results do not agree with his 

 as to Eaja. Torpedo I have not investigated. 



It appears to me probable that the ova in all Elasmobranch 

 Fishes have at some period of their development the two mem- 

 branes described at length for Scylhum. Of these the inner one, 

 or zona radiata, will probably be admitted on all hands to be a 

 product of the peripheral protoplasm of the egg. 



The outer one corresponds with the membrane usually regarded 

 in other Vertebrates as a chorion or product of the follicular 

 epithelium, but, by tracing it back to its first origin, I have been 

 led to reject this view of its nature. 



T/ie follicular ejnlkelium. — The follicular epithelium in the 



