124 GERM-CELL CYCLE IN ANIMALS 



that this phenomenon does not occur in all insects and 

 that we must seek some larger cause than the un- 

 equal distribution of chromatic elements. 



If no differential divisions are present, as in 

 Dytiscus, what is the cause of the formation of 

 oocytes and nurse cells ? Govaerts decides that since 

 the ultimate oogonium possesses a definite polarity 

 marked by the localization of the "residu fusorial," 

 and the two kinds of daughter cells arise from op- 

 posite ends of the mother cell, the cause of the differ- 

 entiation resides in the polarization of the oogonium. 

 He does not, however, account for this " polarite pre- 

 differentielle." 



Haecker (1912) has described in Cyclops and 

 Diaptomus a three-cell stage in the development 

 of the gonad which is brought about by the delayed 

 division of one of the germ cells of the two-cell 

 stage, and concludes that as in Dytiscus there must 

 be an internal difference in the cells to account 

 for this condition. 



Wieman (19106) has followed the history of the 

 oogonia in Leptinotarsa signaticollis through the 

 larval and adult stages, but was unable to find any 

 evidence that the nuclei inaugurate differentiation 

 as in Dytiscus. He concludes that "the process 

 seems to be the result of several distinct cell elements 

 which operate together as a whole" (p. 148) and that 

 the semi-fluid matrix which results from the lique- 

 faction of cells at the base of the terminal chamber 

 may exert a " specific effect on those germ cells 

 coming under its influence, enabling them to develop 



