648 JOHN BEARD, 



VIII. The earliest History of the Germ-Cells in Embryos. 



Much time and material and considerable pains have been de- 

 voted to the interesting, but difficult, task of tracing the germ-cells 

 to the earliest phases in the formation of the embryo. 



Apart from Eigenmann, who records their presence in a Tele- 

 ostean fish, Cymatogaster, before any mesoblastic somites are formed, 

 the investigator, whose account of them goes to the earliest period, 

 is Prof. C. Rabl. He found them in Pristiurus in an embryo of 

 18 somites. 



Judging from the few — barely half a dozen ^) — embryos of 

 this form in my collection, Pristiurus appears to be a favourable ob- 

 ject for their study. In these the germ-cells seem to be fewer in 

 number than in the skate, and they exceed the ordinary somatic cells 

 in size to a greater degree than in the latter 2). 



In Scyllium canicula, in which this contrast is not so marked, 

 the first germ-cells can be traced to embryos of 12 somites. Their 

 number, however, is very small. For work of this sort Scyllium is 

 more favourable than Raja, because the yolk in its somatic cells be- 

 comes diminished, and, perhaps, disappears, much sooner than in Kaja. 

 Indeed, the circumstance, that the yolk in the somatic cells of Raja 

 is as abundant as in the germ-cells in early embryos, i. e., those with 

 open or closing-in medullary folds, renders it impossible to distinguish 

 the one sort of cell from the other by this character. By careful 

 measurements of cells it would, I am convinced, be possible to find 

 distinctions. As will appear anon, in still earlier periods, as well as 

 in later ones, the germ-cells are larger than those engaged in form- 

 ing the embryo. Such measurements have not yet been carried out 

 in blastoderms, in which there was any degree of embryo-formation- 

 They would, it will be admitted, be rather tedious to make. 



When the attempt to trace them in, i. e., within, the very earliest 

 embryos of Raja failed ^), attention was directed to the later phases 



1) In 1900. 



2) Since writing the above the average total of 127.3 germ-cells 

 has been found in 13 embryos of Pristiurus. In the same form the 

 germ-cells vary in size between 0.018 and 0.02 mm. Balfour ('78, 

 1, p. 131) found their sizes to be 0.016 to 0.036, but he does not 

 name the species to which these measurements refer. 



8) Failed, because a thing cannot be found in a certain place, 

 when it is not there! In a later section it will be demonstrated, that 



