SPECIAL PROBLEMS OF MITOSIS 39 



the salivary gland nuclei (about 65,000// ^) is natural if 

 they are polyploid and the thickness of the chromo- 

 somes results from the fact that they have divided 

 a large number of times. 



The formation of the chromocentre has recently 

 been shown ^^^ to be due to the fact that the regions 

 immediately adjace ntto the spindle attachment 

 (which arp inert but all 'homologous') have, as a 

 result of their homology, all fused into a single mass 

 (Fig. 7d). The origin of the chromocentre is thus 

 ultimately the result of the somatic pairing pheno- 

 menon. Diptera which lack a chromocentre pre- 

 sumably lack the homologous regions next to the 

 spindle attachments. 



Sex-Chromosomes 



All chromosomes other than sex-chromosomes are 

 called autosomes. The sex-determining mechanism 

 of the majority of bisexual animals and plants con- 

 sists of a pair of chromosomes which may be regarded 

 as modified autosomes. In one sex these form an 

 equal pair of homologous chromosomes while in the 

 other the pair is unequal. The sex which possesses 

 the unequal pair is called the heterogametic sex since 

 it produces two kinds of gametes or spores ; the other 

 sex is called the homogametic sex. In most groups of 

 organisms it is the male which is the heterogametic 

 sex (that is to say there are two kinds of sperms or 

 pollen grains and only one kind of egg or megaspore), 

 but in some groups it is the female which is hetero- 

 gametic — that is to say there are two kinds of eggs 

 or megaspores, all the sperms or pollen grains being 

 alike (see Table II). In the Bryophyta (mosses and 

 liverworts) where the sexual stage of the life cycle 

 is haploid the male and female gametophytes each 

 contain one member of a pair of sex-chromosomes. 



It will be seen that the sex-determining mechanism 

 is merely a special kind of heterozygosity — in respect 

 of a whole chromosome instead of in respect of a 



