112 POPULAR SCIENCE MONTHLY. 



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 tliis 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 cer- 

 tain 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 fertiliza- 

 tion 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 condi- 

 tions, 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 con- 

 ditions are exactly the same as when a female spermatozoon enters a 



