18 Studies in the Hybrid Bistoninae. IV 



in all probability, the active agent in procuring the sex rerersal. In 

 other words, we* have to seek for some event which, at one and the 

 same time, dislocates the sex system and likewise brings into being 

 circumstances ending in a half-sized imago. 



Suppose the female-producing ovum which gave rise to this insect 

 to have been fertilised by a spermatozoon containing, as a result of an 

 ordinary reduction division, one X chromosome or gene. In composition 

 the zygote would be represented as X Y, for we must not lose sight of 

 the fact that, in spite of the zygote's being a male, in these hybrids 

 it carries the sex chromosomes typical of a female when found in pure 

 species. Imagine that, in the first cleavage, there is a mitotic dislocation 

 leading to the passage of an undivided X chromosome to one pole and 

 of a like unsplit Y chromosome to the other, this happening owing to 

 their failing to take part in the usual division. Then the cell on one 

 side endowed with the X chromosome would, if viable, tend to induce 

 the appearance of male characters on that side whilst that on the other 

 would, again if viable, by virtue of its unique Y chromosome, end in the 

 appearance of female structures. Thus, if both survive, the outcome 

 would of necessity be a lateral gynandromorph, probably both genetically 

 and somatically. 



If, on the contrary, a cell of the type XO cannot exist, then we have 

 left a cell of composition YO, obviously of female possibilities, to con- 

 tinue the perfection of the organism. That such a destruction of one 

 of the first cleavage cells does not imply a suspension of development 

 but rather ends in the appearance of beings of inferior size has been 

 proved often enough by the artificial removal of such a cell. 



This differential viability of cells of the types XO and YO has been 

 definitely proved by Bridges' cytological investigations as to the chromo- 

 somes of certain flies in his Drosophila cultures, known to be of untypical 

 chromosome content. He found, however, that it was the XO zygotes 

 that could continue to develop whereas the FO's were incapable of 

 existence. We must note, however, that his experiments involved 

 Diptera in which the females are sex homozygotes and the males, on the 

 other hand, heterozygotes — exactly the reverse to what obtains in the 

 Bistoninae. This assures us that, in our case, the preferential survival 

 may be in favour of the cell YO. Incidentally, it indicates that the 

 function of the Y chromosome is not that of a mere foil to the X 

 chromosome in the mechanism of the reduction division, but that, as 

 demanded by the potential theory of sex, it has a real positive and 

 functional value in the matter of sex determination. 



