The determination of sex in animal development. 713 



male, and 511 in the female smooth skate, JR. haiis. The possible 

 existence of other numbers is not to be understood as here excluded. 



8. From the facts concerning the number of primary germ-cells, 

 and from certain other factors, to be afterwards referred to, it may 

 be concluded, that sex is actually differentiated and decided during 

 oogenesis. The facts point to the last division of the oogonia, 

 and the formation of oocytes, as the particular epoch i), at which 

 this happens. That is, the oocytes are differentiated into two cate- 

 gories, destined to become male- and female-eggs respectively. 



9. In the male, as the researches of Meves^) and other factors 

 render evident, a corresponding differentiation of the direct forerunners 

 of two sorts of gametes may happen at the like period, at the final 

 division of the spermatogonia into spermatocytes ^) (Fig. B, page 750). 

 But the second form of male gamete is not now of functional value. 

 In other cases, as in the skate, the complete formation of a second 

 form of gamete apparently does not come about. 



10. Underlying the phenomena of sex, therefore, there are three 

 sorts of functional gametes : in some, possibly in very many, instances 

 a fourth sort of gamete is differentiated in the male, but in the 

 latter never more than one kind of gamete is of functional import. 



11. In parthenogenesis the sex of the "offspring" is not in any 

 sense a consequence of the non-fertilisation of the egg. It would be 

 more correct — at any rate in many cases, thus in the bee and 

 plant-louse — to say, that the non-fertilisation of the egg was the 

 final result of the production and maturation of either male- or 

 female-eggs. 



12. The phenomena of parthenogenesis depend upon the acqui- 

 sition of the faculty of developing without fertilisation on the part of 

 either male-eggs, or female-eggs, or of both male- and female-eggs. 



13. Whenever in parthenogenesis long series of forms of the one 

 sex appear from unfertilised eggs, the eggs, destined to form the other 

 sex, must have been either delayed in their ripenings, or suppressed. 



It must also be stated with great emphasis, that the degeneration 

 and death of germ-cells, not only in pre-embryonic and post-embryonic 

 life, but also in that of mature organisms, is a phenomenon constantly 

 happening. 



As one instance of this sort of elimination of germ-cells the 

 following case from Haecker (in: Zool. Jahrb., V. 5, Anat., p. 219, 



1) Compare, however, Tigs. A and B and Section VIII. 



2) These are described in a later section (page 733). 



