THE MORPHOLOGY OF THE CHROMOSOMES 129 



Lewitsky to the view that in various tribes and genera there has been a 

 progressive shortening of one arm in many of the chromosomes, primitive 

 members of a group having mostly isobrachial chromosomes and derived 

 members mostly heterobrachial ones. In Drosophila the idiograms 

 in the various species bear a general resemblance to one another but 

 show characteristic minor differences-^ (Fig. 71). Even within a Linnsean 

 species the idiograms may not be alike in all individuals. Thus in 

 Rumex acetosa the plants fall into several classes on the basis of the 



H ir 11 H 



p D. melanogaster 6? O.Simulans O.virilis 



#;.% 



4^4^ 





O.funebris 0- willistoni o D. obscura <? 



Fig. 71. — The somatic chromosome complements of several species of Drosophila. 

 Pairs known to be sex-chromosomes shown in solid black; those presumed to be are marked 

 with cross lines. {After Metz, Moses and Mason, 1923.) 



karyotype. Such cytological studies have now been made on a con- 

 siderable number of genera-'' and are to receive further consideration 

 in Chapter XX. 



Groups of related genera, as well as the related species of a genus, 

 often have the same general karyotype. For example, in Haworthia, 

 Gasteria, and Aloe the complement consists of four long and three short 

 pairs, with minor peculiarities in each genus" (Fig. 72, E to G). A 

 notable example of similarity of chromosome complements throughout 



25 Metz (1914, 1916), Metz and Moses (1923), Metz, Moses, and Mason (1923), 

 Frolowa (1926), Frolowa and Astaurow (1929). 



"^^E.g., Vicia (Sweschnikowa, 1927a6, 1928, 1929; Heitz, 1931; Senjaninova- 

 Korczagina, 1932); Trilicujn (Kagawa, 1927a6, 1929a6c); Pellia (Heitz, 1928a6); 

 jEgilops (Senjaninova-Korczagina, 1930); Mecosfethus (McClung, 1928ab); Rumex 

 (Kihara and Yamamoto, 1931); Nigella, Delphinium, Aconitum, and other Helle- 

 borese (Lewitsky, 1931c). 



27 W. R. Taylor (19256). See also Darlington (1926) on Scillete, Langlet (1928) 

 on Berberidaceae, Kachidze (1929) on Dipsacaceae, Tschechow (1930) and Tschechow 

 and Kartaschowa (1932) on Leguminosie, and Saez (1928) on mammals. 



