480 ROBERT W. HEGNER 



or just after the eggs are fertilized, or, in parthenogenetic forms, 

 shortly before maturation and cleavage are inaugurated. 



The localization of the Keimbahn-determinants at the time of 

 their appearance seems to be predetermined. In insects the 

 posterior end of the egg is invariably the place where these 

 bodies occur. In species whose eggs undergo total cleavage they 

 are, as a rule, under normal conditions, segregated in one definite 

 blastomere from the two-cell stage up to the thirty-two cell 

 stage, and are then distributed among the descendants of the 

 single primordial germ cell. In Ascaris it is normally the cell 

 at the posterior (vegetative) pole that fails to undergo the diminu- 

 tion process. It seems therefore that there must be some mechan- 

 ism in the egg which definitely localizes the Keimbahn-deter- 

 minants. 



The segregation of these substances in one blastomere at the 

 first cleavage division is a result of their previous localization, 

 but in later cleavage stages events are more difficult to interpret. 

 Both Haecker ('97) and Amma ('11) have attempted to explain' 

 the distribution of the 'Ectosomen' in copepods by postulating 

 a dissimilar influence of the centrosomes resulting in the segre- 

 gation of these granules at one end of the mitotic spindle in the 

 dividing stem-cell. According to Zeigler's hypothesis, the cen- 

 trosomes during unequal cell division are heterodynamic, and 

 Schonfeld ('01) believes that the synizesis is due to the attraction 

 of the chromosomes by the centrosomes. It is well known that 

 in many cases where unequal cell division occurs, one aster is 

 larger than the other, and this may be the true interpretation 

 of the phenomena, but to the writer it seems more probable that 

 the entire cell contents undergo rearrangement after each cell 

 division, possibly under the influence of the material elaborated 

 within the nucleus and set free during mitosis. Elpatiewsky 

 ('09) also believes in the unequal attractive force of the centro- 

 somes in Sagitta, as indicated in the following quotation: 



Nach der vierten Teilung kommt der besondere Korper in dem 

 Wirkungskreis eines Zentrosomas, namlich desjenigen, welcher naher 

 der Polarfurche liegt. Fast die ganze 'Energie' dieses Zentrosomas 

 wird fiir die Ueberwindung der Vis inertiae des besonderen Korpers 



