THE MUTATED GENE 51 



monsters, then, may be explained on the basis of different degrees 

 of inhibition, acting upon different stages in development and 

 lasting different times. The action of the mutant genes is 

 conceived, as mentioned above, as a general inhibition, to which 

 the regions most active at a particular moment are the most 

 susceptible. It might be debatable whether this special inter- 

 pretation is correct (a description in terms of organ-forming stuffs 

 or organizers or determination stream could easily replace the 

 description in terms of gradients; (see Lehmann)). But the 

 general conclusion is certainly correct, which Wright formulates 

 thus: "The case illustrates the extent to which morphological 

 changes of an apparently qualitative sort may be brought about 

 as a result of ramifying correlative effects from initial gene effects 

 of a simple quantitative sort." We shall realize when discussing 

 the formation of pattern that this result furnishes another 

 example for the general correctness of Goldschmidt's views on 

 primary pattern formation as controlled by genes. 



The facts described in this chapter may be linked with the 

 contents of the next chapter by some facts discovered recently by 

 Bonne vie (19366). She studied the development of still another 

 hereditary abnormality in mice, pseudencephaly, caused by a 

 recessive gene and consisting of a complete upset of the arrange- 

 ment of the brain which rides like a wig on top of the head. Again, 

 it was found that the primary cause of the defect occurs very 

 early in development. The decisive point is that the develop- 

 ment of the primary head mesoderm is affected. It seems that 

 all individual developmental processes are perfectly normal but 

 that the relative velocities of growth have been changed: the 

 ectodermal head parts are growing while their mesenchyme 

 surroundings are still underdeveloped, which forces the former to 

 fold and to behave abnormally. The rest of the abnormal 

 development is the mechanical consequence of this primary 

 situation. In other words, the effect of the mutation is to destroy 

 the proper timing of two major interdependent developmental 

 processes by changing the rate of only one of them. 



3. THE MUTANT GENE AND THE RATE CONCEPT 



The facts reported thus far all point in one direction : they show 

 that the mutant gene produces its effect, the difference from the 

 wild type, by changing the rates of partial processes of develop- 



