170 WILLIAM WALLAOR. 



division ; none of the ova had as yet entered upon their 

 period of growth. In embryos 45 mm. long, however, 

 permanent ova as large as '08 mm. in diameter were present. 

 IMiese ova had evidently been growing for some time, so that 

 it is fair to assume that the ova for the first reproductive 

 period commence to grow in the ovary of the foetal fish when 

 the latter is between 30 mm. and 40 mm. long. 



Observations mentioned above indicate that these first 

 formed oogonia, or more correctly a certain proportion of them, 

 ripen within two years, because certain if not all individuals 

 of this species are mature at the end of their second year. 



In post-embryonic ovaries of Zoarces oogonia are found 

 either singly in the epithelium or collected into '^ nests" just 

 below the epithelium. Such '^ nests" were found in ovaries 

 of adolescent and adult females in the months of February, 

 May, June, July, August, September, and October. In the 

 month last named embryos were present in the ovary. 



The oogonia and developing oocytes in Zoarces are 

 localised in certain roughly longitudinal tracts of the inner 

 surface of the ovary, " ovigcrous lamellse," found in so many 

 other teleosteans, being in this species suppressed. There is 

 no obvious difference, in regard to the histological characters 

 of the cells, between the epithelium covering the ovigerous 

 tracts and that overlying the intermediate barren areas. 



Where oogonia occur singly they are always found in the 

 epithelium intercalated between the ordinary cells of this 

 layer. At this stage the sex cells do not differ, so far as 

 could be made out, from the epithelial cells, except in being 

 slightly larger and rounded. The fact that the youngest sex 

 cells (oogonia), when they occur singly, are never or rarely 

 found anywhere but in the epithelium appears significant. 

 On the hypothesis that the sex cells are formed once for all 

 in the embryo, and never do novo in the ovary, one would 

 expect occasionally to find single oogonia in the subjacent 

 stroma, since, if these special cells are simpl}' stored up in 

 the ovary, there is no apparent reason why they should 

 always be confined to the epithelium. 



