The determination of sex in animal development. 741 



be supposed to influence the destiny as to sex of the offspring, the 

 embryo. 



The idea, that in some mysterious way or other the sex of each 

 individual embryo may be determined in a particular direction during 

 the development, is doubtless responsible for failure to appreciate the 

 actual facts. 



The main source of error has, of course, lain in the futile search 

 for a determination of sex during and in the course of the development 

 of some particular individual, long after it had really been decided. 

 As we have seen, sex is not determined during this, but it is already 

 predestined for the next generation long before this actually begins 

 to arise. 



Were there some hidden process at work during the development 

 — daemon in embryone — two sorts of gametes, spermatozoa and 

 ova, might suffice. Their office would merely be that of giving origin 

 to an embryo of no predestined sex, or, as some embryologists still 

 believe, one whose nature was in the meantime hermaphrodite. A 

 theory of sex of this kind would seem to postulate some hidden 

 genius in the development. Moreover, it would not furnish any ex- 

 planation of the facts. 



If, on the other hand, sex be really predestined, the question raises 

 itself "how does this come to pass, in what way is the sex of an 

 embryo, ultimately destined to develop, decided before even the egg, 

 set apart to furnish it, has yet reached maturity?" 



Where the individuals are all alike, as in the majority of the 

 Protozoa, either like gametes or two sorts of unlike ones are sufficient. 

 If the gametes become unlike, this may ultimately lead to the pro- 

 duction of unlike individuals. The case of Voriicella with two sorts 

 of unlike gametes but with like individuals shows, that this does not 

 of necessity follow from the former facts. If the individuals become 

 unlike, i. e., of two kinds, this will entail other conditions upon the 

 gametes, from whose union these arise. The conjugation of two sorts 

 of gametes is not calculated to bring about the production of two 

 kinds of individuals ^). At least three categories of gametes are 



1) Ordinary hermaphroditism illustrates the truth of this, for here 

 with two sorts of gametes, (female-)eggs and spermatozoa, but one sort 

 of individual, the hermaphrodite, arises. The commonly accepted view 

 of the primary nature of hermaphroditism — what Fritz Müller ironically 

 described as "die Ursprünglichkeit der Zwitterbildung" — is a complete 

 travesty of the facts ; so much so, that one may repeat Fritz Müllek's 



