DIFFERENTIAL BEHAVIOUR 



313 



(c) Trisomic supernumerary fragments in Fritillaria imperialis, 

 one of which must often be entirely unpaired, show no evidence 

 of precocity (D., 1930 c), while supernumeraries in Metapodius, 

 which may be present to the number of six and must be of common 

 origin and homologous (Wilson, 1909), show strongly differential 

 condensation. 



(d) Precocious condensation is not usual in sex chromosomes in 

 the homozygous sex, e.g., in Orthoptera (Mohr, 1915), but it has 

 been found in male Natrix and Gecko (Reptilia) and female Nycieretttes 

 (Mammalia) which are homozygous (Minouchi, 1929 ; Nakamura, 

 1931). It does not occur in Enchenopa (Homoptera), Lebistes 

 (Pisces), or in angiosperms where the dissimilar sex chromosomes 

 pair and cross over (Winge, 1923, 1930 et al.). 



(e) Frequently the precocious autosomes in the Orthoptera 

 form unequal bivalents, one of them being deficient {e.g., Brachysfola 

 and Arphia, Carothers, 1913 ; McClung, 1927. Chorihippus 

 parallelus, Janssens, 1924 ; Belar, 1929 ; D., 1936). It has been 

 thought by Belar and others that their precocity was due to their 

 inequality. But the causation is, in fact, the other way round. 

 They are precocious also when they are equal. It is their genetic 

 property. It is also presumably their genetic property that their 

 deficiencies do not matter because what is lost is inert. Hence their 

 frequent inequaUty . The precocity of these autosomes does not show 

 itself until pachytene and therefore does not interfere with pairing. 



It is evident in Mecostethus, Rhahditis and Alydus, on the other 

 hand, that the precocity ot identical chromosomes prevents their 

 pairing. That this should be so follows on the precocity theory, 

 for the earlier development of the chromosomes will compensate 

 for the earlier prophase and restore mitotic conditions. This has 

 been shown in Alydus and Mecostethus where the chromosomes which 

 do not pair are already divided at the time the other undivided 

 chromosomes are pairing. Thus precocity of the chromosomes 

 determines their failure of pairing, not the reverse. The importance 

 of this failure of pairing in the evolution of sex chromosomes and the 

 development of dissimilarities between them will be considered later. 



Conclusion. While it is possible to classify the differential 

 behaviour of chromosomes according to these three types, there 



