HEREDITY 



the organism. Such an idea would be incorrect. The ferti- 

 lized egg has a gene pattern, it is true, and the type of gene 

 pattern which it possesses has a governing influence on the 

 characteristics of the adult; but the fertilized egg also 

 contains cytoplasm, and cytoplasmic activities are by no 

 means to be neglected. Further, a fertilized egg must meet 

 hospitable conditions in order to develop; and these 

 conditions may be varied. And finally, the behavior of the 

 individual which results from the interaction of gene 

 pattern, cytoplasm, and environment, is also conditioned 

 by experience. 



Cytoplasm is the term used for the mass of protoplasm 

 in which the nucleus containing the chromosomes is 

 embedded. Its precise function is not yet wholly clear. 

 Perhaps we should not go far wrong if we considered it to 

 be simply one of the internal factors of environment. It 

 appears to contain no essential substances which, like the 

 chromosomes, must be transferred to the daughter cells 

 at each cell division in order to have the machine function 

 effectively. On the contrary, the cytoplasm seems to be 

 built up anew each time a cell divides, as if it were being 

 manufactured under the direction of the nucleus. Never- 

 theless, it plays an extraordinary part in early development. 

 In certain animals it can be shown that the cytoplasm of 

 the unfertilized egg gradually arranges itself into three 

 strata, and that these strata constitute the groundwork 

 of the new individual. For example, in the sea urchin one 

 layer becomes the body covering, one the inner part of the 

 alimentary tract, and one the skeleton; and if any layer is 

 disturbed, there is a corresponding disturbance in these 

 particular parts. Now this cytoplasmic behavior may be 

 directed by the chromosomes, and there is some evidence 

 that this is so; but at all events, whatever structural 

 differentiation takes place in the unfertilized egg occurs 

 under the sole influence of the genes of the mother. The 



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