THE MIT AT ED GENE 227 



tion closely following Goldschinidt's .suggestions on the basis of a 

 chance distribution of formative materials. 



There should finally be mentioned some recent non-genetic 

 experiments by Darby (1934) on the old problem of asymmetry 

 of claws in Crustacea. Przibram (1901) had shown that, after 

 removal of the large snap claw, at the next molt the pinch claw 

 produced a snap claw, and a pinch claw was formed from the 

 stump of the old snap claw. Much work has been done since 

 on the same and similar problems, without leading to an under- 

 standing of the processes involved. Darby removed first one 

 claw and then the other after different time intervals. Thus, he 

 found the critical time (about 30 to 42 hr. after the first operation) 

 at which the fate of the regenerate will be decided. By proper 

 experimentation he could thus shift the process so that on both 

 sides pinch claws or on both sides snap claws or on both sides 

 intermediates were formed. He refers this change of asym- 

 metry into symmetry to the relative times of production and to 

 the quantity of two morphogenetic substances A and B. The 

 factors controlling them he calls generally environment and points 

 out that here environment supersedes the effect of the genes. 

 From this he concludes that the genes are responsible for the 

 production of the substances, and the environment for their 

 quantity. If we substitute for environment the combination of 

 integrating processes, all of which are gene controlled in normal 

 development and all of which may be shifted by external agencies, 

 we come again to the point of view regarding symmetry and 

 asymmetry that we have developed above. 



Ludwig (1936) has discussed the same experiments and 

 attempts an explanation which is different from Darby's (and a 

 similar one by Dawes, 1934). His argument is as follows. The 

 claws have hereditarily an alternative norm of reaction (pinch or 

 snap). In regeneration, the decision falls in favor of the alter- 

 native for which at this moment the larger quantity of determin- 

 ing material is available (see Lymnaeus, page 224). This 

 material is being increased because its production is controlled 

 by the type of claw that is developing. A formal explanation 

 for the rest of the experiments may be derived this way. Also, 

 this point of view may be expressed in the terms developed above. 

 But as no genetical facts are known that might be linked with the 

 experimental results, this short review may suffice. 



