CLEAVAGE AND DIFFERENTIATION 



act by removing impediments to cytoplasmic reactions. 

 Let us examine this proposition more closely, 



I begin with the assumption that with the onset of the 

 fertilizable condition of the egg there are in the cytoplasm 

 all the potencies — chemical reactants — whence arise the 

 future organs. Indeed, as experiments on fragments of 

 eggs obtained during this period show, such cytoplasm has 

 capacity to produce several embryos. The chromosomes 

 in such an t^g now begin to remove from the cytoplasm 

 potencies so that others remain free to initiate reactions 

 responsible for differentiation. Thus progressively restric- 

 tion ensues. 



Contrast this condition with that in the history of the 

 spermatozoon. With the two rapidly ensuing maturation- 

 divisions the cytoplasm of the spermatocyte is divided 

 among four spermatids giving rise to mature spermatozoa 

 with less cytoplasm than the mature Qgg possesses. Egg 

 and spermatozoon thus are markedly different with respect 

 to the amount of their cytoplasm at the moment of fertiliza- 

 tion. We may recall a further difference mentioned earlier 

 in this book. Whilst eggs can develop in some cases with- 

 out the spermatozoon, no spermatozoon has ever been 

 found capable of development without the cytoplasm or 

 at least a fragment of the cytoplasm of an egg. In 

 capacity for development, thus, the egg is superior to the 

 spermatozoon. This superiority is related to the egg's 

 cytoplasm since this can develop either with egg- or sperm- 

 nucleus. Now, according to our conception, the egg- 

 nucleus gives up its potencies to its cytoplasm at some 

 moment before the fertilizable stage. The sperm-nucleus, 

 being likewise capable of taking up potencies again in the 

 development of the Q%g which it fertilizes, must have lost 

 its own potencies also before it takes up those residing in 

 the egg-cytoplasm. When this loss occurs we do not know. 

 A probable assumption is that as early as the moment when 



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