344 EDWIN G. CONKLIN 



is forced into a meridional position so that eight macromeres 

 are formed, each of these will give rise to three micromeres, just 

 as each of the four macromeres does in the normal egg. If con- 

 versely the second cleavage furrow is suppressed, but the nuclear 

 division is not, each of the two daughter nuclei in each macro- 

 mere may divdde in such a way as to give off the regular number 

 of micromeres from these two macromeres. 



All this proves that the formation of micromeres at the third 

 cleavage is not due to the segregation of a peculiar 'micromere 

 substance' at the animal pole, for in whatever plane the first tw^o 

 cleavages may divide the egg or in whatever axis the egg substances 

 may be displaced by centrifugal force, each macromere, if freed 

 from external pressure, still give rise to a typical micromere at 

 the third cleavage. The localization and orientation of the mi- 

 totic spindle, which determines the size' and position of the cleavage 

 cells depends upon the viscid spongioplasm or kinoplasm which 

 connects nucleus and centrosphere with one another and with the 

 peripheral layers rather than upon the more movable constituents of 

 the cell: and furthermore, since the orientation of the spindle dif- 

 fers ifi successive cleavages in a characteristic manner, the orien- 

 tation of this viscid protoplasm must also differ. 



Consequently, we may conclude that there is an inherent col- 

 location of spongioplasm in every cell which determines the for- 

 mation of micromeres at the third cleavage, and this does not 

 occur before the third cleavage, since the differentiation of spon- 

 gioplasm has not proceeded far enough at the first and second 

 cleavages to make possible the formation of micromeres. 



I have shown elsewhere ('12) that if a blastomere is isolated in 

 the 2-cell stage, the second cleavage is normal and gives rise to 

 two typical macromeres; if one is isolated in the 4-cell stage, the 

 three following divisions of that cell produce micromeres just 

 as in a normal egg. But no normal micromere can ever be gotten 

 from a cell before the period of the third cleavage. In short, the 

 organization of the kinoplasm in a blastomere of the 4-cell stage 

 differs in some essential way from that in an earlier or later stage, 

 — it has reached a certain peculiar stage of differentiation. Di- 

 rect observation of eggs in various stages from the time of fer- 



