> THE MIT ATED GENE 209 



time when the pattern of the tarsus in the leg disks is laid down 

 by a process of rhythmical subdivision of the distal end of the 

 Anlage, something, an evocator, is present in the germ that 

 diffuses into the Anlagen of the segmental appendages to produce 

 this result. Disks in the proper stage of development will 

 react to this induction by tarsus formation. Normally, the 

 antennal disk is far behind in differentiation at this time and is not 

 influenced by the evocator. The mutant gone, which speeds up 

 antennal differentiation, however, makes the antennal disk 

 mature simultaneously with the leg disks; and the evocator 

 substance, which "orders" the formation of tarsus segments, 

 therefore also acts in this disk. Here we have a case where a 

 simple shift in the time element of gene action results auto- 

 matically in a complicated morphogenetic change. Such facts 

 show a system of genie action at work which we have discussed 

 in a former chapter as the system of reaction velocities in tune. 

 Here we may finally draw attention to the great evolutionary 

 significance of such facts in connection with the problem of single 

 large steps, the problem of the "hopeful monsters" as Gold- 

 schmidt (1933c?) has called it, which is much more serious than 

 this terminology would indicate. 



C. Genes and Pattern 



The facts just described justify the conclusion that the process 

 of embryonic stratification is controlled by the action of the genes. 

 To gain further insight we must, however, turn to more easily 

 accessible forms of pattern, which may serve as models from 

 which generalized conclusions upon all kinds of pattern may be 

 drawn. 



Different types of pattern are available for such studies. 

 As the most elementary type may be considered the simple 

 growth pattern, i.e., the pattern-like typical differences in growth 

 in the three dimensions which lead to the establishment of a 

 definite form of all developmental units. Much work has been 

 done in this line to establish the general laws of differential 

 growth, to mention only D'Arcy Thompson (1916) and J. S. 

 Huxley (1932). The insight that these authors gained has 

 to be linked with specific genie action which might set in motion 

 the pattern of differential growth. 



