y^ MITOSIS : THE VARIATION OF THE CHROMOSOMES 



Kostoff, 1929) with other species (« = 12), of Cardamine pratensis 

 with other species (Lawrence, 1931 d), of Orgyia thyellina with 0. 

 antiqua (Cretschmar, 1928), and of various species of Gryllus 

 (Ohmachi, 1929), indicates similar processes in the ancestry of these 

 forms. 



Within species " fusion " may be inferred in races of various 

 species of Hesperotettix, Mermiria and Jamaicana (McClung, 1914, 

 1917, Woolsey, 1915) and Crepis tectorum. " Fragmentation " 

 may be inferred in Vicia Cracca (Sweshnikova, 1928, and Fig. 14), 

 Phragmatohia fuliginosa (Seiler and Haniel, 1921), Gryllotalpa 

 gryllotalpa (de Winiwarter, 1927, Barrigozzi, 1933) and Felis 

 domesticus (de Winiwarter, 1934). 



More complex changes, such as inversion and interchange, 

 necessarily cannot be inferred from a comparison of mitosis in 

 different species. They will be considered in relation to meiosis 

 and crossing over. 



When larger groups are considered a more generalised effect is 

 seen. Thus in the ninety-four species of the Lepidoptera examined 

 the greatest number of species is found with thirty-one chromosomes, 

 and the frequency with other numbers diminishes regularly with 

 increasing remoteness from thirty-one. This indicates, as Beliajeff 

 (1930) points out, that the group has arisen from ancestral forms 

 with this number. The variation in number has arisen by random 

 fragmentation and fusion. The fragmentation and fusion account 

 for certain differences in size of the chromosomes also, but not for 

 more than a small part. 



Thus Dasychira pudihiinda {n — 87), may be supposed to be 

 derived by fragmentation from a Lymantria-like type with thirty-one 

 chromosomes, for while most of the chromosomes are reduced in 

 size, three pairs remain thirty or forty times the bulk of the rest. 



On the other hand, where two species in the same family like 

 Spilosoma luhricipeda and Miltochrista miniata have the same 

 chromosome number and the chromosomes correspond in size- 

 variation in each, but are throughout forty times larger in one 

 species than in the other, probably genotypic control of chromosome 

 size is responsible for the difference (c/. Beliajeff, 1930 ; D., 1932 a). 

 A fortiori when between two species in the same family of plants 



