414 F. M. BALFOUR. 



ova in the poat-embryonic periods. Ilis views will be best ex- 

 plained by quoting the following passage : — 



"The follicle of Skates and Dog-fish, with the ovum it con- 

 tains, is to be considered as an aggregation of the cells of the 

 single-layered ovarian epithelium which liave grown into the 

 stroma, and of which one cell has become the ovum and the 

 others the follicular epithelium. The follicle, however, draws in 

 with it into the stroma a number of additional epithelial cells 

 in the form of a stalk connecting the follicle with the superfi- 

 cial epithehum. At a later period the lower part of the stalk 

 at its junction with the follicle becomes continuously narrowed, 

 and at the same time a rupture takes place in the cells which 

 form it. In this manner the follicle becomes at last constricted 

 off from the stalk, and so from its })lace of origin in the super- 

 ficial epithelium, and subsequently lies freely in the stroma of 

 the ovary." 



He further explains that the reparation of the follicles from 

 the epitlielium takes place much earlier in Acanthias than in 

 Raja, and that the sinkings of the epithelium into the stroma 

 may have two or three branches each with a follicle. 



Semper gives very little information with reference to the post- 

 embryonic formation of ova. He expresses his agreement on the 

 whole with Ludwig, but, amongst points not mentioned by 

 Ludwig, calls attention to peculiar aggregations of primitive ova 

 in the superficial epithelium, which he regards as either rudi- 

 mentary testicular follicles or as nests similar to those in the 

 embryo. 



My observations on this subject do not agree very closely with 

 those either of Ludwig or Semper. The differences between us 

 partly, though not entirely, depend upon the fundamentally dif- 

 ferent view we hold about the constitution of the ovary and the 

 nature of the epithelium covering it (vide p. 389 and 390). 



In very young ovaries (PI. XVII, fig. 8) nests of ova (in my 

 sense of the terra) are very numerous, but though usually su])er- 

 ficial in position are also found in the deeper layers of the ovary. 

 They are especially concentrated in their old position, close to 

 the dorsal edge of the organ. In some instances they do not 

 present quite the same appearance as in the embryo, owing to 

 the outlines of the ova composing them being distinct, and to 

 the presence between the ova of numerous interstitial cells derived 

 from the germinal epithelium, and destined to become follicular 

 epithelium. These latter cells at first form a much flatter folli- 

 cular epithelium than in the embryonic periods, so that the 

 smaller adult ova have a much less columnar invesfme it than 

 ova of the same size in the embryo. A few })rimitive ova may 

 still be found in a very superficial position, but occisionally 



