The germ-cells. 657 



close of the segmentation, or to the period immediately preceding the 

 first formation of the embryo, their actual presence in earlier phases 

 has not yet been established by observation. He found them in an 

 embryonic foundation, in which no mesoblastic somites were yet pre- 

 sent; ray observations have revealed them still earlier, before there 

 is any real trace of the future embryo. In this respect, 

 apart from others, the present work may claim to be an advance. 



Whenever the segmentation of the egg of Raja shall have been 

 described and figured after the manner of Rückert's work ('99) on 

 Pristiurus and Torpedo, it will be a raatter of comparative ease to 

 determine the particular period of the egg- cleavage, at which they are 

 separated ofi'. But it may be surmised, that in Raja they go back, 

 as in Micrometrus and Cyclops, to a very early period. From their 

 average sizes no certain conclusion can be drawn in the absence of 

 information as to these early cleavage phases. 



But in embryo No. 454 their number was found to be 512, in 

 embryo No. 448 there were 490 germ-cells without counting a con- 

 tingent of about two dozen others in an impossible situation, and 

 none of the observations in earlier embryos of less than 32 mm 

 (No. 454 measured 32 mm and No. 448 26 mm) are at variance with 

 this. Obviously 512 represents either 8 or 9 divisions. 



KüCKERT ('99) has shown, that with the 10th synchronous division 

 the segmentation-cavity is formed. Beyond this point our knowledge 

 is indefinite, but, judging by the numbers and sizes of the cells at 

 the close of the cleavage in the skate, it appears to me unlikely, 

 that the majority of the cells divide more than twice or thrice 

 after the formation of the segmentation-cavity, and before the period 

 represented by embryo No. 434, where the formation of the embryo 

 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 i) as Eigenmann ('92) 

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

 the case in Cyclops. 



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

 presented 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 



1) This has now (Dec. 1901) proved not to be the exact epoch. 

 The primary germ-cells do undoubtedly go back to a certain well-defined 

 period of the cleavage, as will be shown in detail in Part III. 



