870 THE PHYSIOLOGY OF REPRODUCTION. 



that there are male and female eggs to begin with, and that 

 the determination of sex resides in the maternal organism alone. 

 Some of the facts that support this view with more or less con- 

 elusiveness are as follows: (1) In certain worms (Dinophilus) eggs 

 of two sizes are produced; the large eggs on fertilization develop 

 always into females, the small ones into males. Similar facts are 

 recorded for other animals (Hydatina). (2) Many species of in- 

 vertebrates exhibit the phenomenon of parthenogenesis, that is, 

 the eggs of the mother develop without fertilization. In some 

 cases this method forms the only means of reproduction, and the 

 individuals of the race are all females. But in other animals re- 

 production is effected either by parthenogenesis or by fertilization 

 according to the conditions, change of seasons, etc. Among 

 these latter animals it may be shown, in some cases at least, that 

 the parthenogenetic eggs may give rise either to males or females, 

 a fact which accords with the hypothesis of the existence of male 

 and female eggs in the mother. (3) In man twins may be born. 

 These twins may be of two kinds. First, those that are developed 

 from two different eggs, each of which has its own chorion and 

 develops its own placenta. This kind may be designated as false 

 twins, and in the matter of sex they may be male and female, 

 or both male, or both female. The matter varies as in the statis- 

 tics of births in general. In the other group, however, of true 

 twins or identical twins, the two embryos are developed from a 

 single ovum and are included in a single chorion. In such cases 

 the sexes of the twins are always the same, they are both boys 

 or both girls. This fact speaks strongly in favor of the view that 

 the sex may be predetermined in the ovum, which may be either 

 male or female. However, if we grant the fundamental fact, so 

 for as the ova are concerned, that they are either male or female 

 at the time of formation or are made so during the process of growth 

 and maturation, it is still logically possible that there may also be 

 male and female spermatozoa, and that in the union of the two cells 

 the sex of the fertilized ovum may be referable either to the ovum 

 or spermatozoon. It is not justifiable to assert that the paternal 

 organism is entirely without influence upon the sex of the offspring. 

 In fact, in the case of honey bees it is observed that if the egg of 

 the queen bee is unfertilized it develops into a male, but, if fer- 

 tilized, into a female, thus indicating a determining influence upon 

 the part of the male element. If, on the basis of this fact, we 

 assume the existence of male and female eggs and male and female 

 spermatozoa the question of the sex of the offspring would seem 

 to depend upon which of the sex-determining structures in the 

 two cells predominates after union. Our problem still remains 

 unsolved, although reduced to a narrower field for observation 



