CLEAVAGE AND DIFFERENTIATION 



dealing here with this conversion of cytoplasm into nuclei 

 we are concerned with the formation of a known chemical 

 stuflf. If now we relate differentiation to this chemical 

 process, a process whereby a definite chemical stuff is 

 removed from the cytoplasm so that cytoplasmic reactions 

 are rendered possible, we may have close at hand an 

 explanation of differentiation as a series of chemical changes. 

 In this wise it may be possible to detect certain chemical 

 stuffs as potencies. Surely, the more we can substitute for 

 such terms as "potency" chemical reactants in chemical 

 reactions, the closer shall we come to the solution of the 

 problem of differentiation of development. In relating 

 differentiation in part to the synthesis of nuclear material, 

 we take the first step in this direction. 



The progressive differentiation of the egg during cleav- 

 age according to this conception is brought about neither 

 by the pouring out of stuffs by the chromosomes into the 

 cytoplasm nor by segregation of embryonic materials as 

 postulated by those who uphold the theory of embryonic 

 segregation, but by a genetic restriction of potencies 

 through the removal of stuff from the cytoplasm to the 

 nuclei. 



Consider a fertilized egg, ABCD, with determinate cleav- 

 age, which at first cleavage forms two blastomeres, AB and 

 CD; there must be differentiation, since the AB and the 

 CD blastomeres when separated give rise to partial larvae. 

 This would mean according to the theory of segregation 

 that AB is minus CD's material and CD is minus AB's. 

 AB thus would be a cell in which the prospective ^5-poten- 

 cies are present; the same would be true in the CD cell for 

 the CZ)-potencies. After the second cleavage, A-, B-, C- 

 and Z)-potencies would be present in blastomeres A, B, C 

 and D respectively. Similar segregation would happen at 

 each succeeding cleavage. For eggs with indeterminate 

 cleavage the presence of material — and conversely, the loss 



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