NATURE OF THE GENETIC EFFECTS 459 



other parts of the body, has been carried out with other characters, 

 inchiding those associated with a difference in sex. In some cases the 

 technique has involved the breakage and loss of an entire chromosome by 

 treatment either before or after fertilization; ring chromosomes are 

 especially suitable for this purpose. 



Inasmuch as the inhibition, by radiation, of those processes of growth, 

 differentiation, and regeneration which require cell division is probably 

 caused by the damaging action of structural changes of chromosomes on 

 the cells descended from the irradiated ones (as noted in Sect. 10), any 

 morphogenetic or other developmental studies employing radiation in 

 this capacity constitute illustrations of one type of use to which the 

 genetic effects of radiation are put for the investigation of developmental 

 processes. This method has proved a fruitful one in the hands of experi- 

 mental embryologists, especially when the application of the radiation 

 has been limited in space and time to certain parts and stages (e.g. 

 blastemas), whose influence on a given developmental reaction can 

 thereby be traced. The method is likewise useful in the study of some 

 physiological processes of the adult which depend upon the proliferation 

 of given cells (e.g., those of the hematopoietic system), since it makes 

 possible the study of the consequences of reducing the effective numbers 

 of these cells. 



Potentially by far the most analytical use to which the production of 

 genetic changes by radiation may be put in studies of developmental 

 processes is through its pro^'ision of mutant genes, the effects of which on 

 development are then traced in detail. A great many studies of the 

 ways in w^hich given mutant genes influence development — a field of 

 ingestigation known as ''developmental genetics" — have been carried 

 out in Drosophila and other small organisms commonly used in genetic 

 work, and a few, of considerable interest, in mice and poultry. Although 

 hitherto genes which arose by spontaneous mutation have usually been 

 employed, it is to be expected that, with the increasing use of radiation to 

 produce mutations, in higher as well as in lower forms, the genes which 

 are obtained in this way will furnish an ever larger portion of the material 

 for such work. The field is a virtually unlimited one since, theoretically, 

 the method could be applied for each of the thousands of different genes 

 capable of mutating, and even for each of the different mutant alleles 

 of these genes. Moreover, it can be used for gene combinations, in the 

 study of the effects of gene interactions, as well as in combination with 

 varied environmental conditions and artificial techniques. In fact, in 

 the face of this overwhelming wealth of possibilities, the greatest prob- 

 lem may be the proper selection of those types of gene effect for study 

 which involve the more basic and significant processes. For although a 

 complete knowledge of developmental reactions requires the eventual 

 study of all gene effects, it is evident that the study of most of these 



