376 PERMANENT HYBRIDS 



seen lying on opposite sides of the plate (Roller, unpublished ; 

 Plate XI). Their behaviour is like that of paired chromosomes 

 which are driven by overcrowding to form accessory plates (Ch. 

 XII). The regularity in lying off the plate in the Orthoptera is 

 due to the univalence of the X chromosome, and perhaps also to 

 its lower surface charge. The regularity of X and Y in lying at 

 opposite sides of the plate in the Neuroptera is due to mutual 

 repulsion in a restricted space, following a similar distribution at 

 diakinesis. 



Other special properties of sex chromosomes seem to require no 

 other explanation than precocious division of the centromeres 

 where a univalent divides at the same time as the bivalents, and 

 terminal affinity acting at a distance where X and Y pair 

 momentarily at metaphase. The difference in time of division of 

 unpaired X chromosomes in Protenor, Chorthippus, Photinus and 

 Vandiizea is strictly comparable to that of other unpaired chromo- 

 somes in hybrids and mutants of plants. Univalents, as a rule, 

 vary in their timing relationship, in some animals dividing only 

 at the second division (probably Chorthippus, D., 1936 h) and in 

 the extreme opposite case (in Culex, Moffett, 1936) scarcely 

 lagging behind the bivalents at first anaphase (contrast Schrader, 



1935). 



(iv) Haplo-Diploid Sex Determination . (a) General, In several 



groups of Metazoa sexual females give female offspring when their 

 eggs are fertilised, male when they are not fertilised, as first shown 

 in the bee by Dzierzon. This is believed to be the method of sex 

 differentiation in the Hymenoptera, Rotifera, and in some 

 Thysanoptera, Acarina and Hemiptera. In the Rotifera, genera- 

 tions of females reproducing by diploid parthenogenesis can be 

 intercalated between the sexual generations, as in the Aphides. 

 In the hymenopteran Neuroterus certain females lay only fertilised 

 eggs and others only unfertilised. 



It has been shown in several of these groups that the females are, 

 in fact, diploid and the males haploid. The evidence for this 

 method of sexual differentiation, especially in Thysanoptera and 

 Rotifera, is chiefly from breeding results. But in the following 

 instances the assumption of male haploidy has been verified 



