THE BIOLOGY OF THE CELL SURFACE 



develops throughout this period without cytoplasmic 

 division. The furrowing of the cytoplasm, that is the act 

 of cleavage itself, has been thought of as the cause of 

 differentiation. This assumption however, is untenable, 

 as I shall now show. 



By means of cleavage the cytoplasmic mass, the egg, 

 is successively sundered into smaller parts, the blastomeres. 

 This sundering, even if regarded as a means for the separa- 

 tion of constituents, does not bring about a new distribution 

 of them by which arise embryonic parts. True, the divi- 

 sion into smaller parts will facilitate distribution, but it 

 can not be its cause. It is not the mere division into cells 

 that makes the head, neck, arms of a human being or that 

 distinguishes the liver as an appendage of the gut from 

 which it is derived. Rather, concomitantly with the 

 act of cleavage changes take place by which the embryonic 

 areas subsequently arise. 



From yet another consideration we conclude that cleav- 

 age as such is not differentiation. Even if we should 

 assume that the uncleaved egg were an undifferentiated 

 system, cleavage-partitions would only set off undifferen- 

 tiated areas and the differentiation which arises during 

 cleavage would be due to some other cause — as for instance, 

 chemical reactions induced in a previously homogeneous 

 mixture by the act of cleavage in setting up separate reac- 

 tion-chambers of minute capillary dimensions. But these 

 reactions and not the act of cleavage would then be the 

 cause for differentiation. Or were we to suggest that by 

 the establishment of cell-boundaries brought about by 

 cleavage, the increase of surface renders possible surface- 

 reactions characteristic of capillary spaces, we should still 

 not support the proposition that the mere act of cleavage in 

 sundering the protoplasmic mass is responsible for the pro- 

 gressive differentiation which can be followed throughout 

 the cleavage-process. 



2g8 



