334 



PRINCIPLES OF EMBRYOLOGY 



reveal the 'poiiit of attack' of a gene, but can as a rule tell us little about 

 the gene's mode of operation or the time of its primary activity. 



The sensitivity of a stock of animals to environmental stimuli is under 

 genetic control. One can, by selection over a number of generations, 



Figure 15.2 



The curves above show the frequencies per cent of various types of pheno- 

 copies produced when a temperature shock (3-5 hrs. at 39° C.) was given at 

 certain times after puparium-formation to a stock of D. inelanogaster. The 

 type of wings corresponding to the various curves are shown below, to- 

 gether with the wild type ( + ). They are : (a) first broadened type ; {h) dumpy 

 type; (c) first narrowed type; (d) rounded end type; [e) second broadened 

 type, usually curved as well ; (/) normal proportioned but small ; (g) second 

 narrowed type. (From Schatz 195 1.) 



