FRAGMENTATION OF CHROMOSOMES 135 



somes (Fig. 61, B, C) including the fourth. Translocations involv- 

 ing the second and third are the most numerous apparently, either 

 chromosome being the donor. (We do not intend to enter into 

 the genetic data here as they are irrelevant to the cytology except 

 as confirmatory evidence.) 



Another modification induced by X-ray application is known 

 as " deletions." These occur in the X-chromosomes (Fig. 61, D). 

 Many cases are known involving a large or small portion of the 

 chromosome and differences in the transmission of the sex-linked 

 characters have been observed in these cases as well. 



The importance of these discoveries from the point of view of 

 the cytologist lies in the fact that the possibility of chromosome 

 fragmentation and fusion with other chromosomes is now demon- 

 strated and can be used as a working hypothesis in treating the 

 chromosome groups of related species. 



For instance, the work of Metz (83) has shown, a good time ago, 

 that the chromosome groups in various species of Drosophila 

 could be accounted for on the assumption that either fragmenta- 

 tion or fusion of chromosomes had taken place. This has now 

 been demonstrated as a fact in X-radiated forms. 



It must be remembered that these cytological observations are 

 all made from embryonic or somatic tissues and not from ger- 

 minal. This does not invalidate the results at all, but it certainly 

 puts a limit to the possibilities of cytological confirmation of the 

 behaviour during germ-cell formation in the maturation divisions. 

 Several early attempts were made to investigate spermatogenesis 

 in Drosophila, e.g., Stevens in 1908. These were more or less un- 

 successful owing to the difficulty of handling the material 

 adequately. (The gonads are surrounded with fatty tissue, which 

 prevents the penetration of most good cytological fixatives.) 



Metz has, however, recently given a description of spermato- 

 genesis in Drosophila funebris, which species he found the most 

 favourable (84). Synapsis apparently occurs between homo- 

 logous chromosomes in the telophase of the last spermatogonial 

 division. The sex chromosomes remain relatively condensed and 

 attached to or incorporated in the nucleolus. In the early growth 

 phases of the spermatocyte the autosomes become so diffuse that 



