CLEAVAGE AND DIFFERENTIATION 



tion of the cytoplasmic materials by the nucleus takes place 

 with each mitotic cycle. 



We know, as has been pointed out, that as cleavage pro- 

 gresses the nuclei increase in number and en fnasse in vol- 

 ume. That is, they grow. We know also that in this same 

 period the total volume of cytoplasm decreases and that the 

 growth of the nuclei is at the expense of the cytoplasm. 

 Genetic restriction then depends upon the removal by the 

 nucleus of certain materials from the cytoplasm, leaving 

 others free. The free materials determine the character of 

 the cell — as ectoderm, mesoderm or endoderm and later as 

 any one of the organs arising therefrom. 



Something leaves the cytoplasm with each cell-division; 

 then why not the materials of the cytoplasm which are no7i- 

 specific for the given blastomere .? Certainly, the mecha- 

 nism for the removal of material is present in this growth 

 of the amount of nuclear substance as development pro- 

 gresses. With each cleavage each nucleus fixes all material 

 other than that which makes the blastomere what it is, 

 AB or CD; A or B, C or Z), etc., to the end of cleavage. 

 Then the nucleus of cell AB is different from that of the 

 CD cell since the ^5-nucleus contains bound CD-material 

 and vice versa for the CD-nucleus. 



Then the potencies for embryo-formation are all present 

 in the uncleaved tgg\ cleavage serves to remove these and 

 this removal is fast or slow, early or late depending upon 

 the species of egg. The question arises: When does the 

 cytoplasm of the egg gain its potencies for differentiation 

 during cleavage t 



Pluripotency becomes demonstrable through merogony, 

 as was pointed out, with the onset of the fertilizable condi- 

 tion inasmuch as now fragments of the egg are when fer- 

 tilized each capable of development. From this we might 

 reason that the potencies that were stored up in the nuclei 



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