SPECIAL PROBLEMS OF METOSIS 81 



decondensation of the heteropycnotic sex-cliromo- 

 somes between the two divisions. It results in the 

 chromosomes orientating themselves in the long 

 axis of the second division spindle, above and below 

 the equatorial plane, so that one goes to each pole 

 (cf. Anasa, Alydus and Protenor, where there is a 

 similar mechanism but without a Y). 



In organisms with an X^X^Y mechanism (e.g. 

 the Praying Mantis) each X has a pairing region 

 homologous to a corresponding pairing region in the 

 Y. No region in X^ is homologous to any part of 

 X^, so that X^ and X^ never pair together but 

 always with different parts of the Y.^^ The converse 

 situation occurs in Rumex acetosa where Y^ and Y^ 

 pair separately with parts of the X.^^^ Thus in 

 both cases a sex-trivalent is formed with its three 

 component chromosomes held together by chiasmata. 

 In Mantis X^ and X^ go to the same pole at the 

 first division, Y going to the other pole, in Rumex 

 acetosa Y^ and Y^ go to one pole and X to the other. 



Meiosis in Haploid Organisms 



In Insects with male haploidy it is usual for the 

 first meiotic division to be entirely suppressed so 

 that the sperms are formed with the same number 

 of chromosomes as the somatic tissues of the adult 

 male. Vestiges of the first meiotic division do occui*, 

 however, in most cases. Thus in the male Bee 

 (drone) the prophase stages of the first meiotic 

 division take place, but the metaphase and anaj)hase 

 are entirely omitted ; the second meiotic. dixision 

 then follows ^^^ ; in some other liaploid insects the 

 first division is even more completely sui)pr(^sscd.^^ 

 In plants, on the other hand, where haploid sporo- 

 phytes occur occasionally, both meiotic divisions 

 usually occur. In ' true ' haploids all the chiomo- 

 somes behave as univalents, dividing in either the 

 first or the second division, but never in both. 

 Some haploid plants, however, have small sections 



6 



