414 ROBERT W. HEGNER 



male germ cells corresponding to the formation of nurse cells 

 in the cycle of the female germ cells; and (4) to test Wieman's 

 ('10b) statements regarding the occurrence of amitosis in the 

 oogonia and nurse cells, and in the spermatogonia during the 

 formation of testicular cysts. 



(1) The differentiation of nurse cells and oocytes in the ovary of 

 Leptinotarsa decemlineata. So far as we know, in the majority 

 of cases the cells which are segregated as primordial germ cells 

 in the embryo do not produce eggs or spermatozoa. Many of 

 them degenerate, although they iDrobably once possessed the 

 potentiality of true germ cells; others undergo modifications, 

 becoming nurse cells, Sertoli cells, etc. 



Anaong the most interesting cases of the differentiation of 

 nurse cells from germ cells is that of Dytiscus marginalis. We 

 owe detailed accounts of the process in this species to Giardina 

 ('01), Debaisieux ('09) and Giinthert ('10), but as long ago as 

 1886 Korschelt figured what was evidently one stage rn this 

 differentiation. Giardina ('01) estabUshed the fact for Dytiscus 

 that the mitoses which result in the formation of nurse cells are 

 differentiating, as theoretically postulated by Paulcke ('00). 

 During the four divisions preceding the formation of the oocyte, 

 a single oogonium gives rise to one oocyte and fifteen nurse 

 cells. A differentiation takes place in the chromatin of the 

 oogonial nucleus, one half consisting of a condensed mass, the 

 other half of large granules which correspond to the forty chro- 

 mosomes of the oogonium. During mitosis the chromosomes 

 become arranged as an equatorial plate, and the chromatic mass 

 forms a ring about it — the 'anello cromatico.' This ring passes 

 intact to one of the daughter cells, whereas the chromosomes 

 are equally divided. During the succeeding mitoses similar differ- 

 ential divisions occur, resulting in one oocyte containing the chro- 

 matic ring, and fifteen nurse cells lacking this nuclear substance. 

 Thus, as Paulcke's theory demands, the difference between the 

 nurse cells and the oocytes is the result of internal and not 

 external causes. 



Giardina ('01) considered the formation of the chromatic 

 ring as a sort of synapsis, and later ('02) distinguished between 



