658 



THE AMERICAN NATURALIST 



[Vol. LI 



determined, as it does in certain other insects, is unknown. 

 It is thus certain that polarity exists soon after the be- 

 ginning of the growth period and that bilaterality is prob- 

 ably also established at an early stage. 



The differentiation of oocytes and nurse cells in dy- 

 tiscid and gyrinid beetles is of peculiar interest, although 

 the early and later history of the germ cells in these in- 

 sects is not known. In the diving beetle, Dytiscus mar- 

 ginalis,^^ a single oogonium gives rise to fifteen nurse 

 cells and one oocyte. The oocyte and its mother cell, 

 grandmother cell, and great-grandmother cell can be dis- 



