458 RADIATION BIOLOGY 



tions, some of them very elaborate, which provide precisely designed 

 genetic machinery, of varied types, for use in the more or less automatic 

 carrying out of given genetic operations that reciuire repetition on a mass 

 scale. For example, in the finding of lethal and other mutations in the 

 second chromosome, a task which reciuires breeding as far as the third 

 filial generation, the chief factor which in the past limited the scale on 

 which any such work could be carried out was the necessity for obtaining 

 virgin females of a particular kind from each of the numerous second- 

 generation cultures, for breeding with males from the same culture, so 

 selected as to have second chromosomes of the same kind as those in the 

 females. But nowadays, by the use of a technique involving a specially 

 constructed "sifter stock" (Muller, 1951b), in the production of which 

 radiation was employed but which is too complicated in genetic structure 

 and operation to be described here, all flies of the second filial generation 

 meet with genetic death before maturity except the females and males of 

 the required kind. Thus the females do not need to be obtained as 

 virgins, and the offspring of this generation need merely to be trans- 

 ferred en masse to new cultures, for the production of the third genera- 

 tion; in the latter generation the presence of the mutations being sought 

 for is readily evident on inspection. In such ways, then, the genetic 

 tools provided by radiation have greatly increased the productivity of a 

 given amount of work, especially in the fields of mutation frequency and 

 of the frequency of mutant genes in populations. At the same time they 

 have made it possible to employ, in part of that work, less highly skilled 

 assistants than were formerly necessary for it. 



24-3. Fields of Development, Physiology, Pathology, and Biochemistry. 

 Not only problems of genetics proper and of evolution, but also those of 

 development, have had Ught thrown upon them by making use of the 

 genetic effects of radiation. One such line of attack is concerned with 

 the tracing of the cell lineage of parts of the body, and with the degree of 

 autonomy with which given characters develop. This is well illustrated 

 in Patterson's (1929) experiments in irradiating the embryos and larvae 

 of Drosophila which were heterozygous for the recessive gene for white 

 eye. In these experiments observations of the size, shape and position 

 of the resulting white spots in the eyes of the adult flies — spots now known 

 to have been produced, in the great majority of cases, by somatic crossing 

 over (Muller, 1941; Auerbach, 1945) — showed that the cells of the optic 

 anlagen divide approximately once in 12 hours, up to a given stage. The 

 observations showed, further, that the region of the eye which a given 

 cell is to form is indeterminate except that the descendant cells tend to 

 remain together in a group, and that the pigment develops autonomously 

 in this case, i.e., its development or nonde\-elopment is determined by 

 whether or not the given cell contains the normal allele of white, regard- 

 less of which allele the neighbor cells contain. Similar work, involving 



