CASTLE: THE HEREDITY OF SEX. 205 
TD. ExcrrrionaAL PARTHENOGENESIS IN BOMBYX MORI, ETC. 
Occasional parthenogenesis is known to occur in certain Lepidoptera, 
when the mother is forcibly prevented from copulating. The cases 
which have been most carefully studied are those of the silk moth, 
Bombyx mori, and the gypsy moth, Ocneria dispar. The unfertilized 
as well as the fertilized eggs of these species are known, through the in- 
vestigations of Platner (88) and Henking (’92), to undergo two matu- 
ration divisions. But only an occasional unfertilized egg develops to the 
larval stage, — only one in several hundred, or even one in thousands. 
A still smaller proportion attain the condition of imagos. These few, 
however, are of both sexes, and are capable of reproduction when bred to 
ordinary individuals (von Siebold, ’56). 
But it is entirely possible that in the very exceptional egg which de- 
velops normally, a second maturation division has for some reason failed 
to take place, or after it has taken place, a reunion has occurred of the 
second polar nucleus with the egg nucleus, as sometimes in the egg of 
Artemia, according to Brauer. Such a reunion would bring together 
again the sex-characters segregated in maturation, and would produce 
the physiological and morphological equivalent of the cleavage nucleus 
of a fertilized egg. A similar result would follow the complete sup- 
pression of a second maturation division. 
The occurrence of individuals of both sexes among the partheno- 
genetic offspring of the silk moth and gypsy moth shows that in these 
species, as in other normally dioecious animals, there is no uniform 
dominance of one sex over the other, such as we find occurring among 
normally parthenogenetic animals, where the female character regularly 
dominates. 
V. Abnormal Sex Proportions among Hybrids. 
Bateson and Saunders (: 02, p. 139) consider it as “on the whole 
against the hypothesis that sex depends chiefly on gametic differentiation 
that the statistical distribution of sex among first crosses shows great 
departure from the normal proportions.” The writer does not share this 
opinion, for on the hypothesis of sex advanced in this paper departures of 
the sort indicated are capable of ready explanation. 
It should be stated, however, that the known cases of this sort are 
comparatively rare, whereas the statement of Bateson and Saunders 
might lead one to expect their frequent occurrence. The writer knows 
of but two cases about which our information is full enough to warrant 
statistical examination. 
