34 Studies in the Hybrid Bistoniiiae. IV 



In the maturation divisions of such insects the progress of events 

 is greatly altered. Owing to the fact that chromosomes derived from 

 parents of diverse species are not always homologous, many chromosomes 

 fail to find mates in the first divisions. Then at the point when in 

 normal oogenesis the paired chromosomes separate, to ensure cell division, 

 the unpaired chromosomes in hybrid oogenesis split. Thus, both in the 

 Qgg and in the first polar nuclei, we find varying numbers of chromo- 

 somes which, in extreme cases like ours, attain the full somatic number. 

 Both egg and polar body, in the case of a hybrid female heterozygous 

 for sex, carry a complement of two sex chromosomes. 



Thus, as in the spermatogenesis of hybrid pilzii males, in the 

 oogenesis of hybrid pihii females no genuine reduction division occurs. 



It also appears that, as regards sex chromosomes, the ova of hybrid 

 females agree in being the possessors of two, and thus incapable of 

 yielding on fertilisation zygotes of two types as are the ova of sex 

 heterozygotes of pure species. 



When these abnormal ova are fertilised, as in the case of pilzii by 

 pomonaria, by spermatozoa carrying uniformly an X chromosome, on 

 fusion we get zygotes all alike and of composition XX'Y (using 

 X' to designate the hirtaria chromosome). Two of these, X and F, 

 are inherited from the same species and, being more prone to act in 

 harmony, are likely therefore to be instrumental in settling the sex 

 scheme. The possession of such a pair of sex chromosomes necessarily 

 ends in femaleness. Therefore, the insect will be essentially female 

 but thrown out of equilibrium by the presence of the extra X'. Now 

 the cell containing this combination was built for, and contains the 

 mechanism for dealing with, a normal number of chromosomes but, in 

 place of holding a maximum of, say 100, it is supplied with nearer 116 

 some of which, those of hirtaria origin, are excessively large. 



As a consequence, in the early cleavage divisions and probably also 

 in many instances later, we have abundant scope for a failure of the 

 usual chromosome splitting. Such mitotic dislocation we know to occur 

 but not always in the same form. At one time it may be an X, and at 

 another it may be a F, that lags and passes undivided to one pole, and so 

 on with every possible combination of divided and undivided chromo- 

 somes. As a result of mitotic accidents or incidents of this kind, we 

 shall have produced at varying stages of segmentation cells of varying 

 chromatin content as far as sex (and also other) chromosomes are 

 concerned. If the F goes undivided to one pole, the two daughter 

 cells are of two types, one XX' and the other XX'Y, or, if an X and 



