Expressed in a somewhat more general way, Castle suggests that each egg and each spermatozoon is either a male or a female germ-cell (and not a mixture of the two), and when a female egg is fertilized by a male spermatozoon, or _vice versa_, the individual is a sexual hybrid with one sex dominating and the other latent. The assumption that there are two kinds of eggs, male and female, and two kinds of spermatozoa, male and female, is not supported by any direct or experimental evidence. Moreover, in order to carry out the hypothesis, it is necessary to make the further assumption that a female egg can only be fertilized by a male spermatozoon, and a male egg by a female spermatozoon. While such a view is contrary to all our previous ideas, yet it must be admitted that there are no facts which disprove directly that such a selection on the part of the germ-cells takes place. If these two suppositions be granted, then Castle’s hypothesis is as follows:— In order that half of the individuals shall become males and half females it is necessary to assume that in some individuals the male element dominates and in others the female, and since each fertilized egg contains both male and female elements, it is necessary to assume that either the egg or the spermatozoon contains the dominating element. Castle supposes that in hermaphroditic organisms the two characters “exist in the balanced relationship in which they were received from the parents,” but, as has just been stated, in unisexual forms one or the other sex dominates, except of course in those rare cases, as in the bees and ants, where half of the body may bear the characters of one sex, and the other half that of the other sex. In parthenogenetic species the female character is supposed to be uniformly stronger, so that it dominates in every contest, “for the fertilized egg in such species develops invariably into a female.” Under certain circumstances, as Castle points out, the parthenogenetic female produces both males and females, and this is also true in the occasional development of the unfertilized egg of the silkworm moth, and of the gypsy moth, in which both male and female individuals are produced by parthenogenesis. These facts show that even in unfertilized eggs both sexes are potentially present; but this might be interpreted to mean that some eggs are male and some female, rather than that each egg has the possibility of both kinds of development. If, however, one polar body is retained in these parthenogenetic eggs, then _ex hypothese_ each egg would contain the potentialities of both sexes (if the polar body were of the opposite sex character). It seems necessary to make this assumption because in some parthenogenetic forms males and females may be produced later by each individual, as in the aphids, and this could not occur if we assume that some parthenogenetic eggs are purely male and some female.