i8o 



STRUCTURAL HYBRIDS 



which the chromosome can be broken, corresponds so far as can be 

 seen with the unit of structure, the smallest chromomere visible in 

 ultra-violet photographs, and both are smaller than the unit of 



1^^:^ 



■2^ 



sim mel 



ei-ft /' 



U- 



\ 



-St-- 



fp 



ZBfl 



W,ff 

 t8.0 



sa,s 





fa 



m- 



FiG. 62. — Left : comparative linkage maps of the right arms of 

 the third chromosome in D. melanogastes and D. simulans 

 (Sturtevant, 1929) . Right : pairing of these chromosomes in the 

 sahvary gland of the hybrid between these species. Above ; the 

 two fourth chromosomes of the species. Below : mitotic chromo- 

 somes of D. melanogaster , (J. (Patau, 1935.) 



crossing-over (Ellenhorn, Prokofieva and MuUer, 1935). The total 

 complement of Drosophila melanogaster, which is less than a millimetre 

 long, probably contains over 4,000 of these units. Furthermore 

 Bridges (1935) has been able to show from a detailed description 

 of the whole complement of normal flies that the band-pattern 



