The germ -cells. 649 



of the cleavage, and to the period, when the first traces of an embryo 

 were beginning to be laid down. This was done, in the hope, that 

 something more might be elucidated, or that, at any rate, some start- 

 ing point for a new attack of the problem might be gained. 



The result was far more favourable than might have been anti- 

 cipated. Important new facts were unearthed, but not for a moment 

 will it be maintained, that with them the subject has been completely 

 exhausted in Raja. 



With a considerable degree of certainty the germ-cells of Raja 

 can be carried back to the later phases of the cleavage. Here in the 

 meantime this chapter of their history must be closed. To go back 

 to still earlier phases, to make out definitely, in which of the divisions 

 of the segmenting egg the primary or ancestral germ-cells take their 

 origin, would naturally entail the working-out of the whole cleavage 

 from the beginning. 



I confess to a strong desire to do this. The material is probably 

 partially in my possession ; and, at any rate, no great difficulty in 

 obtaining it would be experienced. To attempt it now would be to 

 delay the publication of the work for some considerable time; and, 

 though the resulting finds might be of general embryological and cyto- 

 logical interest, they might add nothing to the importance of the 

 facts already made out. 



IX. The Grerm-Cells in the Pre-emhryonic Period. 



In the course of the research two facts, not already referred to, 

 pressed themselves into notice. As they afterwards became associated 

 with a third fact, concerning ' a peculiarity of certain cells of the 

 later cleavage, and as they help to elucidate this, they may now be 

 mentioned. 



In some of the germ-cells of certain embryos, thus in that figured 

 in Fig. 10 from embryo No. 410, the nuclei presented a very remark- 

 able doubled appearance. A sight of this was sufficient to recall 

 RtJCKERT's ('95) and Hacker's ('96) discoveries of the persistence in 

 a separate condition of paternal and maternal portions of the nuclei 

 of certain cleavage- cells. Until this was encountered in the skate no 

 thought of its possible existence here ever entered my head. It was 



at this and earlier periods the germ-cells are not within the embryonic 

 foundation, but either in the blastoderm, or in early stages of their wander- 

 ings into the embryo. 



