GEOMETRICAL RELATIONS OE CLEAVAGE-EORMS 



Z7l 



to the mesoblast-bands by continued divisions, always in the same 

 plane at right angles to that in which the rudimentary cells are 

 formed (Fig. 174). The cause of the definite succession of equal and 

 unequal divisions is here wholly unexplained. No less difficult is the 

 extreme inequality of division involved in the formation of the polar 

 bodies. We cannot explain this through the fact that deutoplasm is 

 collected in the lower hemisphere ; for, on the one hand, the succeed- 

 ing divisions (first cleavages) are often equal, while, on the other 

 hand, the inequality is no less pronounced in eggs having equally 



A 



B 



Fig. i^a,. — Rudimentary blastomeres in the embryo of an annelid, Ai-icia. 



A. From lower pole; rudimentary cells at e. e\ tlie heavy outline is the lip of the blastopore. 

 B. The same in sagittal optical section, showing rudimentary cell {e), primary mesoblast (il/), 

 and mesoblast-band (w). 



distributed deutoplasm, or in those, like echinoderm-eggs, which are 

 "alecithal." 



Such cases prove that Balfour's law is only a partial explanation, 

 being probably the expression of a more deeply lying cause, and 

 there is reason to believe that this cause lies outside the immediate 

 mechanism of mitosis. Conklin ('94) has called attention to the 

 fact 1 that the immediate cause of the inequality probably does not 

 lie either in the nucleus or in the amphiaster ; for not only the 

 chromatin-halves, but also tJic asters, are exactly equal in the early 

 prophases, and the inequality of the asters only appears as the 

 division proceeds. Probably, therefore, the cause lies in some rela- 

 tion between the mitotic figure and the cell-body in which it lies. 



^ In the cleavage of gasteropod eggs. 



