142 



STRUCTURAL HYBRIDS 



29 chromosomes is probably a fusion or fragmentation heterozygote, 

 and consequently has pairing of groups of chromosomes (Federley, 



1915). 

 The most general type of fragmentation heterozygote is that in 



which one (usually the smaller) product of fragmentation is present 



as a supernumerary or reduplicated chromosome, so that the 



organism is polysomic in respect of the segment concerned. This 



Fig. 47. — :First metaphase in Tradescantia virginiana {^x = 24). 

 a, an exceptional interstitial chiasma in a group of four chromo- 

 somes, due to arrest of terminalisation by change of homology, 

 cf. Ch. XII. h, a normal quadrivalent held together by a 

 quadruple chiasma. c, an association of six major chromosomes, 

 and three homologous fragments (in black), two of them asso- 

 ciated by lateral chiasmata at corresponding points in the 

 major chromosomes. There is again an exceptional interstitial 

 chiasma between two major chromosomes. X 2400. (After 

 D., 1929 c.) 



condition was first shown in Metapodiiis, Diahrotica and other 

 Hemiptera {Wilson, 1905, a and h, 1909). In another hemipteran 

 (Alydtis, Renter, 1930) supernumeraries occur, one of which is 

 homologous with a part of a larger one, while both must be 

 homologous with part of the sex chromosome, since all three may 

 pair. 



Recently large numbers of plants have been found with 

 supernumerary fragments {v. Ch. Ill), and their pairing has been 



