AMEIOSIS IN HAPLOIDS 459 



In haploids derived from organisms with a prolonged diploid 

 generation two types of meiosis are found according as the haploidy 

 is habitual (in certain male Hymenoptera and in coccids) or 

 adventitious (in angiosperms arising by parthenogenesis from 

 diploid parents). In both the chromosomes are contracted as in 

 ordinary meiosis, but, of course, unpaired. In the drone bee 

 (Apis, Meves, 1907) a spindle is formed and the chromosomes lie on 

 a metaphase plate but do not divide ; at one pole an enucleate bud 

 is cut off, a second division spindle is formed and the chromosomes 

 divide this time to give two daughter-nuclei, one of which is extruded 

 as a polar body while the other gives a haploid (unreduced) 

 spermatozoon. 



In the haploid male coccids {I eery a, etc., n = 2 ; Hughes- 

 Schrader, 1930) only one division occurs, and it is not clear whether 

 this corresponds to the first or the second. In the haploid male 

 rotifers {Asplanchna, n = 13 ; Whitney, 1929) it is the second 

 division that is usually omitted ; where an abortive second division 

 occurs spermatozoa are formed with less than the haploid number, 

 and these do not function. The mechanism is apparently imperfect. 



This suppression of meiosis seems to be a sexual characteristic 

 not directly determined by the haploid condition. Diploids 

 sometimes show the male character in the hymenopteran 

 Habrobraeon, and these produce diploid sperm (Torvik, 1931 ; 

 Torvik-Greb, 1935). They have the abortive meiosis characteristic 

 of males. This shows that the mechanism of meiosis is here 

 genetically adapted to give regular failure of reduction. The 

 condition of the adventitious haploids in the angiosperms, on the other 

 hand, is different. The ordinary meiotic mechanism has to deal 

 with impaired chromosomes, and the result is irregular [v. Ch. X). 



Such observations also show that where meiosis takes place at the 

 end of the diploid generation there is a mechanism of division 

 external to the chromosomes and distinct from that of ordinary 

 mitotic division {ef. Ch. XII). In the adventitious haploid 

 angiosperms we see it acting on unpaired chromosomes, for whose 

 division it is unadapted. In the haploid Hymenoptera we see it 

 specially adapted to deal with the situation so as to produce regular 

 results. 



