CLEAVAGE AND DIFFERENTIATION 



mittently present as discrete drops in viable cells having 

 had any one of several kinds of treatment — whose action 

 is not deleterious since one can prove that the cell after 

 treatment returns completely to the untreated state — has 

 far-reaching significance: it becomes an index to the nature 

 and rate of reactions. In both normal egg and blastomeres, 

 the rhythmical appearance of water-drops allows us to 

 correlate their occurrence with the rhythm in a cleavage. 

 DiflFerences in the formation of the water-drops in diflferent 

 blastomeres we may regard as an index to chemical changes 

 underlying the progressive restriction that runs with 

 cleavage. 



With each succeeding cleavage the egg is further sub- 

 divided into chambers of capillary dimensions. At the 

 end of the cleavage-period the total surface-area of the 

 blastomeres is much greater than that of the uncleaved egg. 

 Thus, during cleavage the ectoplasm increases in amount. 

 This increase of ectoplasm constitutes the last of the six 

 enumerated changes that occur during cleavage. The 

 cleavage-period may be defined as one in which the cyto- 

 plasm diminishes through its conversion on the one side 

 into new nuclear substance and on the other into new cell- 

 partitions. Having examined the role of nuclear growth 

 in differentiation, I seek now to evaluate the evidence 

 from which I derive a postulate concerning the role of that 

 part of the ground-substance located at the cell-surface, 

 the ectoplasm, in differentiation. 



In the first line stands the fact that the ectoplasm 

 increases in amount with the progressive sundering of the 

 egg into blastomeres, regardless of the type of cleavage by 

 which this result is reached. Whilst no animal egg is a 

 perfect sphere and never by cleavage gives rise to blasto- 

 meres which are perfect spheres, nevertheless the ratio of 

 total surface to total mass which obtains for a sphere when 

 sub-divided into spheres is closely approximated by the Qgg 



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