GENES, MOLECULES AND PROCESSES 



Ephrussi and Beadle were able to show by transplantation experi- 

 ments that some of the necessary substances are produced on the 

 spot, and others are brought in from elsewhere. 



Finally we must notice again that the majority of gene changes 

 do not entirely stop or block a process; they merely slow it down. 

 They are hypomorphs rather than amorphs. Indeed they often 

 change a rate of activity in a way that can be exactly measured. 

 The change S-s in the shrimp Gamrnarus chevreuxi reduces to half 

 the rate of deposition of melanin in the eyes of the young larva. 

 The rate falls off with growth, but more rapidly in the quickly 

 depositing type so that in the end the product is not very different. 



A difference in rate may appear as a difference in time of expres- 

 sion. For example in some local races of the moth Lyniantria dispar 

 Goldschmidt found that the skin of the caterpillar has pigment 

 throughout development; in others deposition begins half-way 

 towards pupation, and even in these proceeds at different rates in 

 different races. With the more highly integrated processes, for 

 example, of sexual differentiation, all differences of races and 

 hybrids become referable, as Goldschmidt has shown, to differences 

 in rates of co-operating and competing processes common to all 

 races and both sexes. 



In these terms we begin to see the material and mechanical basis 

 of differentiation. The actions of genes depend on the simultaneous 

 actions of one another. But even more, and somewhat differently, 

 they depend on what other genes have done earlier and have done 

 elsewhere. Even if the immediate products of a gene's action were 

 always constant in amount — which is unlikely — its derivative 

 effects would depend on when and where it was acting, on conditions 

 in the nucleus, and, of course, in the cytoplasm too. 



These considerations bring us back to the cell as the sphere of 

 action of the nucleus. If the cell feeds the nucleus and the nucleus 

 feeds the cell, the separation of cells gives each of them its specific 

 type of feeding relationship, its own pattern of reactions and 

 development. Between the fertilized egg, and its mature develop- 

 ment, the whole course is determined by the supply of raw materials, 

 the rates of gene action and the rates of diffusion of its products. 

 If the supply of raw materials never changed, or if the rates of gene 

 action never varied, or if diffusion were instantaneous, differentiation 



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