The dcterminatiou of sex in animal development. 751 



can happen. I now fall back upon the division of the primary germ- 

 cells into secondary ones as the point, at which the first step in the 

 determination is taken. The initiation of the process here would 

 account for the inability of the latter to undergo independent devel- 

 opment. Moreover, it would explain how in the leech in certain gonads 

 only (female-)eggs were produced, in others the equivalents of male- 

 eggs, spermatocytes etc. ^). Many other developmental facts are cleared 

 up by this reference, such as long successions of eggs of a certain 

 category in some instances of parthenogenesis. 



In the diagrammatic Fig. A an attempt has been made to depict 

 what must be the actual course of events in the oogenesis of Baja hatis. 



To Fig. A another of the spermatogenesis of cases with two 

 forms of spermatozoa has been added in Fig. B. This will complete 

 the comparison, but a lengthy description of the figure will hardly 

 be called for. As oogenesis and spermatogenesis must and do follow 

 along parallel lines, the details of Fig. B closely resemble those of the 

 preceding figure of oogenesis. 



Lastly, the course of events in hemaphroditism is depicted in 

 Fig. C. 



It will, I trust, be clear to the reader how in oogenesis, as 

 depicted in Fig. A, along with the degeneration and suppression of 

 germ-cells anywhere along either of the lines leading to male and 

 female oocytes. Nature is able to regulate the sexes in any way she 

 finds fit. The two means adapted to carry out this end are variation 

 in the number "2ii" and degeneration of germ- cells. In the skate the 

 number "2ii" is apparently greater by a unit along the line of the 

 male oocytes than along that of the female ones. In cases of partheno- 

 genesis with complete suppression of the males "2n" need be only 

 1 along the male line, and even this may as a rule be suppressed. 



It must be pointed out, that the like is true of spermatogenesis. 

 Although in some or many instances, as apparently would be the 



1) The instance of the leech or earthworm would seem to suggest 

 the probable production of only one sort of egg in any particular gonad 

 of a segmented animal, such as a dioecious worm. In all probability 

 the existing arrangement of the gonads of the common leech, for example, 

 has been inherited from some dioecious ancestor, in which the re- 

 presentatives of the present ovaries gave rise to female-eggs, the equi- 

 valents of the existing testes being the seat of "origin" of male-eggs. 

 It may be concluded, that in any segmental arrangement of ovaries in 

 dioecious Metazoa certain of these give rise to male-eggs, others to 

 female eggs only. 



