CLEAVAGE AND DIFFERENTIATION 



with a corresponding chromosome that has previously 

 entered that nucleus. 



What here is individuality? If I am able to keep alive 

 all the descendants which arise from a single slipper ani- 

 malcule by cell-divisions at the rate of three divisions a day, 

 in seven days I shall have 2,097,152 organisms, each of 

 which has 1/2,097,152 part of the original Paramoecium. 

 Only this fraction, then, of individuality is maintained. 

 And this is a much simpler case than that of individuality 

 of each member in a chromosome-complex from fertilized 

 egg to adult of a complex organism. I think that the 

 question of the persistent individuality of the chromosomes 

 is more than a question of terminology. 



If we turn from the chromosomes to the genes themselves 

 that compose each chromosome, we find the same difficulty. 

 It has been suggested that the gene is a molecule. The 

 two daughter-cells arising from first cleavage would then 

 possess chromosomes made up of half molecules; in the four- 

 cell stage there would be quarter molecules; and in each 

 succeeding division there would be a corresponding reduc- 

 tion of the original gene-molecules found in each chromo- 

 some of the fertilized egg. But in chemistry we know of no 

 fractional molecules. Then, let us start with the adult 

 organism. On the assumption that each gene in each 

 chromosome of every cell of the whale, for example, is a 

 molecule, it is necessary to assume that each gene of the 

 fertilized egg is a complex of molecules whose number 

 equals that of the sum total of all the descendants from 

 this gene which are In each and every cell of the adult. It 

 would be necessary further to assume that the gene-mole- 

 cules vary in size in stages of development from egg to 

 adult. The gene therefore can not be a molecule according 

 to physico-chemical usage. The proponents of the gene 

 theory should be the last to postulate any non-physico- 

 chemical molecule. 



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