142 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



should have been retained if obsolete, and should have shrunk to 

 only one, as soon as regular parthenogenesis commenced. 



The double division must have some meaning, and one 

 which is the same in the formation of both spermatozoa and 

 ova. 



I accept the meaning which has been indicated above, and 

 believe that Henking's observations can be easily brought into 

 accord with the plan of formation of sexual cells observed in 

 other species. Henking looks upon the first division of the 

 mother-cell as a ' reducing division,' the second as an * equal 

 division,' and considers that he uses these terms in the sense in 

 which I have employed them. But this is not quite the case. 

 I understand by a ' reducing division,' one in which the number 

 of ids present in the passive nucleus is reduced to half in 

 each of the daughter-nuclei : I understand by an ' equal division ' 

 one in which each daughter-nucleus is provided with the full 

 number of ids present in the passive nucleus of the mother- 

 cell. In the latter case, the daughter-nuclei will contain similar 

 ids, but, in the former, this can only occur when the ids of the 

 mother-cell are precisely identical. I have never maintained 

 that these two contrasted modes of division must be invariably 

 recognizable and distinguishable by external characters, and I 

 have never identified the chromatosomes of authors with my 

 ancestral units. But only when such an identification is as- 

 sumed does the reduction of the number of ids by one-half 

 (i. e. a ' reducing division ' in my sense of the term) necessarily 

 imply a reduction in the number of chromatosomes as well. 

 The types of ' reducing ' and ' equal divisions,' as I propounded 

 them in 1887 \ are so conceived that the first involves a halving 

 of the number of idants, while the second does not. But 

 I expressly added — ' I do not mean to imply that it is im- 

 possible to imagine any other form in which they [viz. these 

 modes of division] may occur 2.' It then seemed to me that the 

 form of nuclear division which is accompanied by a longitudinal 

 splitting of the idants arranged in the equatorial plate of the 

 spindle, can scarcely be conceived of as other than an ' equal 

 division,' but even then I added the words * as far as I can see I* 

 If we assume the linear arrangement of ids in a single row in 



^ See Vol. I. pp. 366-379, and especially pp. 375-377. 

 2 See Vol. I. p. 375. 2 ggg YqI j p ^75, 



