xn] CRUSTACEA, DTTISCUS 183 



called germ-cell determinants are of very different types in 

 different animals, and that their occurrence appears to be 

 very irregular, since they may be present in one genus and 

 absent in others that are nearly related to it. In some cases 

 the difference between germ-cells and body-cells seems to 

 depend on the presence of chromatin in the germ-cell 

 nuclei which is lost from those of the body-cells, but the 

 extrusion of chromatin, when it occurs, appears to be de- 

 pendent on cytoplasmic factors, and in other animals only 

 the cytoplasmic factor is present and the chromatin con- 

 tent of germ-cells and body-cells is to all appearance alike. 

 And of the real nature of the cytoplasmic factor, hardly 

 anything is known ; its existence is only certain when some 

 portion of protoplasm, or of bodies in it, stains differently 

 from the rest, and the apparently great difference between 

 related species may be due, in some instances at least, to 

 a mere difference of staining capacity of the protoplasm 

 concerned with the differentiation. 



The examples mentioned hitherto have all been concerned 

 with the very early differentiation of germ-cells in the 

 embryo, but other cases have been observed where a visible 

 cytological differentiation occurs at a much later stage. 

 Only two of these will be described. One of the best known 

 is that of the separation of the oocyte from the nurse-cells 

 in the Beetle Dytiscus, described by GIARDINA and subse- 

 quently byDzBAisiEux and by GUNTHERT. After a series of 

 typical oogonial divisions, their nuclei undergo a curious 

 differentiation ; the chromosomes appear in one half of the 

 nucleus while the other hemisphere is filled with staining 

 granules which, according to GUNTHERT, are produced by 

 separation of fragments from the chromosomes, though 

 DEBAISIEUX regards them as of nucleolar origin. The mass 



