658 JOHN BEARD, 



"megaspheres" or large cells of Rückert, and already some of them 

 may exhibit a multinucleated condition. 



Normal germ- cells of this period of about 0.02 mm are depicted 

 in Figs. 49 and 50. They contain much yolk, not represented in the 

 figures, and their nuclei are of the duplicated or twin-character, dis- 

 covered by RÜCKERT ('95) and Hacker ('96) in the germ-cells of Cyclops. 



The germ-cells of Raja are formed, therefore, before the embryo 

 arises, and they have to get into it from the outside, from the yolk-sac. 

 It has been found a matter of great difficulty to trace the first appearance 

 of germ-cells within the embryo. This is largely brought about by the 

 yolk, found at the start in all the somatic cells. The only reliable 

 character has not yet been applied to the earliest embryos, viz., the 

 size of the cells, 0.02 mm — the first somatic cells not exceeding 

 0.0125 mm — possibly, also, the twin-nuclear condition would be 

 decisive. 



At the close of segmentation many of the future germ-cells lie 

 in the segmentation-cavity just beneath the site of the future embryo, 

 and there is no doubt, that they subsequently wander into it. Their 

 wanderings begin, indeed, very early. In B. hatis No. 691, in which 

 only 12 mesoblastic somites are yet formed, there are many germ- 

 cells, not actually within the embryonic foundation, but on their way 

 into it between the layers on the blastoderm. In the growing 

 zone of the developing embryo there are no cells atany 

 time large enough to be the progenitors of the germ-cells. 



Whilst within very early embryos of Raja (12 to 20 somites) 

 germ-cells have few been actually seen, Rabl ('96) has recorded them 

 in Pristiurus with 18 somites, and the writer has seen one or two in 

 Scyllium canicula of 12 somites. 



The first formed or primary germ-cells are seen in many of the 

 figures. Indeed, it would be no exaggeration to say, that almost all 

 the drawings show them; for it is only very late in the development, 

 that activity sets in among the primary germ-cells, and their number 

 becomes increased. 



Balfour ('78. 2) distinguishes between "primitive ova" and "per- 

 manent ova". My researches amply confirm him, but for obvious 

 reasons the former name must give place to primary germ-cells, the 

 latter to secondary ones. In R. hatis the primary germ-cells persist 

 as such, until the embryo is at least 42 mm, secondary germ-cells 

 are those of older embryos, from my investigations those of embryos, 

 of 54 mm onwards (Fig. 33). 



