124 ^^^ ANIMALS DEVELOP 



we might say, the driUing-machines and lathes with 

 which the animal is made, and the genes would be 

 the particular tools, jigs, and drills which are fitted 

 on to the machines for the actual job on hand. This 

 idea may seem to suggest that the real fundamental 

 thing is the cytoplasm and that the effects of the 

 genes are purely superficial. To some extent this 

 may be true for any one embryo. But one must not 

 forget that the cytoplasm itself has been evolved 

 through a long series of ancestors and has probably 

 been affected by the genes which they contained. 

 To pursue our analogy, the drilling-machines and 

 lathes were shaped by drills and bits and jigs when 

 they were built. Of course, to insist on pursuing the 

 argument ad infinitum leads to a ridiculous question, 

 like asking whether the hen or the ^g'g came first, 

 because finally the gene and the cytoplasm are 

 dependent on each other and neither could exist 

 alone. 



Can genes affect organization centres? The 

 answer seems to be **yes." True, we know very few 

 cases where genes seem to have a direct effect on 

 an evocator. In amphibians the evocator is present 

 in the Q,gg long before it is fertilized and cannot be 

 manufactured by the genes contained in the 

 fertilized ^gg, though perhaps it may be influenced 

 by the genes of the egg's mother. In the insect tgg 

 Seidel showed that the activation centre, which is a 

 sort of evocator, is formed by a reaction between 

 cytoplasm and nuclei, and maybe it is the genes in 

 the nuclei which take part in the reaction; but we 



