The germ-cells. 651 



neath is yolk. Figures of this period have been given by various 

 authors ^). 



RücKERT ('99) has recently surmised, that at the close of seg- 

 mentation the smaller segments or cells represent the embryonic end 

 of the blastoderm, whilst the larger ones lie in front. This appears 

 to be so in the skate. In the two blastoderms referred to the cells 

 of the anterior end are on the average considerably larger than those 

 at the embryonic end. In the former area measurements show them 

 to vary from 12 to 15 /li, and some are even 20. At the em- 

 bryonic end the majority of the cells are under 10 /n. 

 This is true of practically all the cells forming the "epiblastic" layer, 

 and also of those of the proliferating zone already referred to. 



But within the "germinal cavity" there are other cells, lying 

 immediately under the region of the future embryo. There are 

 usually several of these together, and sometimes they seem to he 

 in rather definite rows. Examples of these are figured in Figs. 49 

 and 50. 



They are larger than any of the other cells of the neighbour- 

 hood, i. e., larger than those just mentioned. Their diameter averages 

 0.02 to 0,036 mm, that is to say their average size is that of primary 

 germ-cells or the immediate forerunners of such. Moreover, though 

 various phases of cell-division are encountered in nearly all the other 

 and smaller cells of this region, they, and they only, are in a state 

 of complete repose. 



Nor is this all. The reader will have already noticed, that these 

 cells present the phenomena of duplicated nuclei. This is true, not 

 only of those figured, but also of the majority. From their size, 

 absolute and relative, from their position and structure, it may and 

 must be concluded, that these cells are germ-cells. 



Whether there be others in the anterior part of the blastoderm 

 is a point not yet satisfactorily determined. It is probably so; for, 

 unlike Rückert ('99), I do not consider the big heap of large cells 

 in this region as largely a store, to be drawn upon, as the building- 

 up of the embryo proceeds. 



Occasionally one meets with multinucleated cells of large size — 

 so-called "raegaspheres" of Kückert — in the embryonic region of 

 the blastoderm, that is, in the "germinal cavity", as recorded by 



1) ZlEGLEK, RüCKEKT, RaBL, HiS, etC. 

 Zool. Jahrb. XVI. Abth. f. Morph. 43 



