The germ-cells. 677 



other words, leucocytes in the skate never possess yolk, nor do they 

 exhibit the phenomena of duplicated nuclei. 



XIV. Primary and secondary (xerm-Cells. 



The present researches have been carried out on a material of all 

 sorts of phases from the close of segmentation to embryos of 42 mm, 

 and they do not extend sufficiently far to permit of the giving of any 

 revised or confirmatory account of the later history, including the 

 details of the formation of the secondary germ-cells. But, as Rabl 

 ('96, p. 754) has already said of his own work, so also it is true of 

 mine, that it overlaps the older researches of Semph:r ('75) and Bal- 

 four ('78). Where my account of the germ-cells ends, there, or even 

 earlier, their history of them begins, and thence, more especially in 

 Semper's memoir, it is carried to a finish in a very complete and 

 masterly fashion. 



There are two points, in which my results come into contact with 

 those of Balfour on later stages. These concern the relations of the 

 primary to the secondary germ-cells and the modes of origin of the 

 latter. 



As a comparison of Figs. 37 and 33 of Plate 44 proves, the 

 primary germ-cells are larger than the secondary ones ^). This statement 

 is in direct contradiction with Balfour's results. In his work on the 

 development of the ovary ('78, 2) he states: "the permanent ova 

 [secondary germ-cells of the present writing] are larger, the smallest 

 of them being larger than the average primitive ova in the proportion 

 of four to three etc.", and in tab. 17, fig. 1 he depicts a transverse 

 section of an ovarian lidge of an embryo of Scyllium canicula stage P. 

 Numerous "primitive ova" are shown — there being no fewer than 22 

 in the section. 



Stage P of Balfour's nomenclature is comparatively late. For 

 other purposes the characters, assigned by him to embryos of different 

 periods, have been collated from the Elasmobranch monograph, and 

 from the analysis it appears, that "stage 0" probably represents Scyl- 

 lium-emhryos of 30 mm of my collection, "stage P" those of 32 — 33 mm, 

 and "stage Q" those of 34 — 36 mm at least. Anything like an exact 

 estimate is difficult, or even impossible; for Balfour gives but few 

 of the characters of his embryos. 



1) The products of the first division of the primary germ-cells, i. e., 

 the first secondary germ-cells, in Raja vary from 0.0126 — 0.014 mm. 



