apply Mendel's law to the problem of sex. Castle is therefore obliged to make a further assumption to avoid this difficulty. He assumes that a male spermatozoon can fertilize only female eggs, and a female spermatozoon only male eggs. There is no evidence known at present supporting this assumption, but it must be admitted that it can not be disproved, however improbable it may appear. On this view every fertilized egg is a sex-hybrid, and may give rise to a male or to a female according to which element dominates. Thus we return once more to our original question as to what determines the sex of the individual. We shall see presently that Castle fails to meet this fundamental question.
There is one result that Castle cites, which he claims indicates that his assumption that the eggs may show a selective power towards certain of the spermatozoa is not unwarranted. He found some years ago in the ascidian, Ciona intestinalis, that the eggs of one individual can not be fertilized by the sperm from the same individual, except very rarely. This case is cited as indicating that successful fertilization depends upon unlikeness between the gametes that unite. I have repeated this experiment on Ciona and have confirmed in large part this result, but, unfortunately for the point of view, I found in other ascidians that this relation does not hold. In Molgula, for example, the eggs are perfectly fertile with sperm from the same individual. Furthermore, by making the sperm of Ciona more active by adding ether to the water, I have been able to make them, under certain conditions, fertilize all the eggs of the same individual. In the light of these facts I do not think the conditions in Ciona can be given the interpretation that Castle has applied to them.
There is another side of Castle's hypothesis that must be briefly referred to, since he suggests a way of meeting a difficulty that is fatal to Beard's theory. I refer to parthenogenetic development and to the production at the end of a parthenogenetic series of male and female individuals. Castle supposes that in parthenogenetic reproduction the female character dominates over the male, when the two are present together, and that when a separation of the sex-characters takes place it does so at the time of the formation of the second polar body in the egg, and probably at the corresponding state of development in the spermatozoon. There is a fact in this connection, the bearing of which Weismann was the first to fully appreciate, namely, that the parthenogenetic eggs of daphnids and of some rotifers give off only one polar body, while eggs that are to be fertilized give off two polar bodies. Castle suggests that the second polar body is the female gamete, hence when it is given off the egg must become a pure male if it develops. If this polar body should be retained in the egg the conditions are exactly the same as when a female spermatozoon enters a
