STRUCTURE AND DE\?ELOPMENT OF VERTEBRATE OVARY. 415 



also in the deeper layers. I am inclined to agree with Semper 

 that some of these are freshly formed from the cells of the 

 germinal epithelium. 



In the young female with ova of about 0*5 mm. nests of 

 ova are still fairly numerous. The nests are characteristic, 

 and present the various remarkable peculiarities already described 

 in the embryo. In many instances they form polynuclear 

 masses, not divided into separate cells, generally, however, the 

 individual ova are distinct. The ova in these nests are on the 

 average rather smaller than during the embryonic ])eriods. The 

 nests are frequently quite superficial and at times continuous 

 with the pseudo-epithelium, and individual ova also occasionally 

 occupy a position in the superficial epithelium. Some of 

 the appearances presented by separate ova are not unlike the 

 figures of Ludwig, but a growth such as he describes has, 

 according to my observations, no existence. The columns which 

 be beHeves to have grown into the stroma are merely trabeculse 

 connecting the deeper and more superficial parts of the germinal 

 epithelium ; and his whole view about the formation of the 

 follicular epithelium round separate ova certainly does not apply, 

 except in rare cases, to Scyllium. It is, indeed, very easy to see 

 that most freshly formed ova are derived from nests, as in the 

 embryo ; and the formation of a follicular epithelium round these 

 ova takes place as they become separated from the nests. A 

 few solitary ova, which have never formed part of a nest, seem 

 to be formed in this stage as in the embryo ; but they do not 

 grow into the stroma surrounded by the cells of the pseudo- 

 epithelium, and only as they reach a not inconsiderable size is 

 a definite follicular epithelium formed around them. The fol- 

 licular epithelium, though not always formed from the pseudo- 

 epithelium, is of course always composed of cells derived from 

 the germinal epithelium. 



In all the ova formed at this stage the nucleus would seem to 

 pass through the same metamorphosis as in the embryo. 



In the later stages, and even in the full-grown female of Scyl- 

 lium, fresh ova seem to be formed and nests also to be present. 

 In Raja I have not found freshly formed ova or nests in the 

 adult, and have had no opportunity of studying the young forms. 



Summary of observations on the develojoment of the ovary in 

 Scyllium and Raja. 



(1.) The ovary in the embryo is a ridge, triangular in 

 section, attached along the base. It is formed of a core of 

 stroma and a covering of epithelium. A special thickening of 

 the epithelium on the outer side forms the true germinal epi- 

 thelium, to which the ova are confined (PI. XVII, fig. 1). 



VOL. XVIII. NEW SER. E E 



