KEIMBAHN-DETERMINANTS 237 



species whose eggs undergo total cleavage they are, 

 under normal conditions, segregated in one definite 

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

 two-cell stage, as a rule, 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 diminution process. It seems therefore that 

 there must be some mechanism in the egg which 

 definitely localizes the keimbahn-determinants. 



The segregation of these substances in one blas- 

 tomere 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 

 (1897) and Amma (1911) have attempted to explain 

 the distribution of the *'Ectosomen" in copepods by 

 postulating a dissimilar influence of the centrosomes 

 resulting in the segregation of these granules at one 

 end of the mitotic spindle in the dividing stem cell. 

 According to Zeigler's hypothesis the centrosomes 

 during unequal cell divison are heterodynamic, 

 and Schonfeld (1901) believes that the synizesis 

 stage 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 elab- 

 orated within the nucleus and set free during mito- 



