GEOMETRICAL KELATIONS OF CLEAVAGE-IORMS 



171 



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, 174. — Rudimentary blastomeres in the embryo of an aiuK-liu. Aruta. 

 A. From lower pole ; rudimentary cells at^. <?; the heavy outline is the lip of tli 

 D. The same in sagittal optical section, showing rudimentary cell (f). piimnv im 

 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 exi>lanati<m. 

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

 :here is reason to believe that this cause lies outside the immediate 

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

 facti that the immediate cause of the inequality j^robably does not 

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

 chromatin-halves, but also tlic asters, are exactly ecpial in the early 

 prophases, and the inequality of the a.sters 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. 



1 In the cleavage of gasteropod eggs. 



