The determination of sex in animal dcTelopment. 757 



•3) As already indicated, the reduction of chromosomes must 

 originally have been merely an undoing of the previous conjugation, 

 such that each separated half became a gamete, or gave rise to such. 

 This separation originally took place at the division of the primary 

 germ-cells into secondary ones. By the intercalation of new mitoses 

 it has been delayed until the end of this intercalated series, until 

 the division of the oogonia into oocytes, for example. The result of 

 this has been, that the reduction is no longer effected by cell-division, 

 but it appears in the closing phases of such a division. 



4) The one half of each nucleus, which originally like the rest 

 itself gave rise to gametes, owing to the separation (Figs. A and B), 

 due to the intercalation of new mitoses, is thus lost in and during the 

 reduction. The separation along two lines to form gametes having 

 taken place once at an earlier point, it cannot be repeated. 



In considering the reduction and allied questions, such as those 

 of heredity, two factors must not be overlooked. These are, that, 

 on the one hand, the line of "ancestry" ends in the "embryo", which 

 forms a termination of two series of "ancestors", paternal and mater- 

 nal, and that, on the other, it is continued onwards for the germ- 

 cells without passing at all through the embryo. This has 

 hitherto, owing to the nature of prevailing conceptions of the rela- 

 tionships of embryo and germ-cells, been ignored. 



Moreover, a reduction in the number of chromosomes is the ultimate 

 consequence of the previous conjugation of two cells. This might 

 seem to be too obvious for mention. But, if it be put in another way, 

 the need of insisting upon it may be clear. As reduction of chromo- 

 somes now precedes conjugation of gametes, it has been assumed, that 

 this was always so, even at the start of the process. Steasburger 

 rightly pointed out some years ago, that the reduction might be re- 

 garded as a reversion to what obtained before conjugation was initiated, 

 and, perhaps, this expresses what has been urged above. 



The result of this is, that in conjugation we can never have a 

 union of half entities or individualities, for the process must originally 

 have started with whole ones, and with such it must be continued. 



This conclusion throws some new light upon Galton's law of 

 ancestral inheritance. 



X. The Regulation of Sex in Nature. 



Properly speaking, the problem of the mode or modes, by which 

 Nature may be supposed to regulate the proportions of the wo sexets 



