656 THE AMERICAN NATURALIST [Vol. LI 



which they reach a high state of specialization. The dis- 

 covery of axial gradients of metabolism in the eggs of 

 certain animals in an anterior-posterior direction^^ sug- 

 gests that this may also be true of insect eggs. If such 

 gradients exist in insect eggs and if the metabolic activity 

 decreases gradually from the anterior to the posterior 

 end, then the primordial germ cells, which arise at the 

 extreme posterior end, are actually the least active meta- 

 bolically of all the cells of the embryo. Their early sep- 

 aration from the egg would also tend to keep them in an 

 undifferentiated condition since they are on this account 

 less likely to be influenced by the rest of the embryo. 



The primordial germ cells in these insect eggs are thus 

 visibly different because of the presence of germ-line de- 

 terminants and are probably physiologically different, at 

 least in part, because of their position at the posterior 

 end of the egg. 



The contents of these cells are as follows (Fig. 4) : 

 (1) part of the cortical layer of cytoplasm, (2) part of 

 the cytoplasm which surrounds the cleavage nuclei and 

 which is collected from among the yolk globules, (3) part 

 of the germ-line determinants, and (4) a nucleus with the 

 full amount of chromatin. The fourth item is mentioned 

 because in Miastor all of the nuclei that form somatic 

 cells undergo a diminution process, being similar in this 

 respect to Ascaris. This chromatin is in Miastor entirely 

 maternal since the eggs of this fly that have been studied, 

 develop parthenogenetically. 



Nothing very definite has been discovered regarding 

 the arrangement of these substances in the germ -cells. 

 The nucleus lies near the center in all of them; the two 

 kinds of cytoplasm soon become indistinguishable; and 

 the germ-line determinants may, at first, be more or less 

 evenly distributed throughout the cytoplasm, as in chry- 

 somelid beetles and Miastor, or may be clumped in vari- 

 ous parts of the cell, as in Chironomus. In every case, 

 however, the germ-line determinants evidently become 



22 Child, 1916, Biol. Bull, Vol. 30. 



