467 



is in initiation. If it be permissible, therefore, to assume the occur- 

 rence of this after the 13th division, the first origin of the germ-cells 

 of Raja would date back to as early a period as Eigenmann ('92) 

 surmised it to do in Cymatogaster, or as Haecker ('96) found to 

 be the case in Cyclops. 



At the close of segmentation the germ- cells of Raja are repre- 

 sented by cells of 0,02 mm, 0,036 mm, and probably by slightly larger 

 products of about 0,05 mm. These latter correspond to the "mega- 

 spheres" or large cells of Rückert, and already some of them 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, and by the former ('99) in the cleavage-cells of Torpedo. 



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 exceed- 

 ing 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 R. batis 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 large enough 

 to be the progenitors of the germ-cells. 



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

 germ-cells have not been actually seen, Rabl ('96) has recorded them 

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

 Scy Ilium canicula of 12 somites. 



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



1) Since the above was written, in May 1900, much greater success 

 has been obtained in this direction. 



30* 



