20 PRINCIPLES OF EMBRYOLOGY 



these segregations is itself influenced by the genes in the maternal ovary 

 in which the egg was formed (see p. 43). The genes can therefore be re- 

 garded as the ultimate controllers of the whole range of developmental 

 processes. 



(2) It is usually held that any given gene only produces one specific 

 immediate effect, although of course from this many secondary conse- 

 quences may eventually follow in later development; and the theory is 

 ofi;en carried a step further by the suggestion that this primary action of 

 the gene is to influence the production of a corresponding enzyme. There 

 is no doubt that much very beautiful work has recently revealed many 

 genes each of which does influence the formation of a particular enzyme. 

 But there is no very compelhng reason to suppose that they do so in a 

 single step, and that this is their primary action; nor can it be shown that 

 all genes influence enzymes; and again it has not been demonstrated 

 that a gene cannot have more than one primary activity, for instance by 

 reacting with different substrates. Indeed, in the present state of our 

 ignorance about developmental processes, it makes very little difference 

 to our general understanding which of these many possibffities we suppose 

 to be true. Any single gene is such a comparatively minor element in 

 the whole complex process of the formation of a tissue or an organ that 

 the general character of its primary action has little relevance at the 

 present time. 



(3) Genetical analysis of well-studied animals, such as Drosophila, has 

 shown that each developmental process is influenced by very many genes. 

 There must be many more ingredient elements in a developmental pro- 

 cess than might be guessed at first sight. We cannot, for instance, hope to 

 give a full account of the development of a nerve cell simply in terms of 

 the synthesis of a single specific nerve protein by a system containing 

 only one or a small number of kinds of molecules ; we shall always be 

 dealing with complex systems containing at least a few tens of different 

 active substances. 



(4) Genetic studies reinforce an important general conclusion which 

 can also be drawn from purely embryological considerations; namely 

 that the reactions between the many substances concerned in a develop- 

 mental process are interlocked so that they become partially self-compen- 

 sating. That is to say, slight changes can be made to the system without 

 producing any effect on the end-result. For instance, most genes show 

 some degree of dominance, which means that when one dominant allele 

 is substituted by a recessive one, little or no difference is made to the 

 animal which develops. Embryologically, we see the same type of pheno- 

 menon when it is found that a normal organ can be formed even if we 



