156 AMPHIMIXIS OR ESSENTIAL MEANING OF [XII. 



more a division which is preceded by a doubling of the 

 idants. 



If this be sOy we cannot doubt that the effect of the process 

 must be similar to that which follows the corresponding 

 processes in eggs which require fertilization, viz. the arrange- 

 ment of idants in fresh combinations) as I attempted to show 

 in the first chapter. We are thus led to the view that in 

 parthenogenetic as luell as in sexual eggs a change may take 

 place in the constitution of the germ-plasm during successive 

 generations. 



If we start from that point in phyletic development at 

 which parthenogenesis was first established, each idant in 

 the original egg-cell was at that time composed of a series 

 of different ids. Then, for the first time, these idants were not 

 diminished to half the total number by two polar divisions, but, 

 after being doubled in the egg-mother-cell and again reduced to 

 half by the first polar division, their number in the mature ovum 

 became the same as in the original egg-cell (see Fig. VIII). By 

 this means a fresh combination was rendered possible and 

 indeed unavoidable, unless we assume that the constituents of 

 each pair of similar idants, which arose from the doubling of 

 the previous idants, separated and united with those of the other 

 pairs, forming two exactly similar groups which then respectivel}^ 

 entered the two daughter-nuclei. This would be the result 

 of an ' equal division ' of the nucleus. Such a division is 

 attained and ensured precisely because the doubling and 

 division of the idants only takes place when they have already 

 become arranged in the equatorial plate ; but whenever the 

 doubling has occurred beforehand, as is the case here, the 

 two halves of an idant may indeed be occasionally shared 

 between the two daughter nuclei, but they may also, just 

 as readil}^, both pass into one and the same daughter-nucleus. 

 From this freedom in the distribution of the idants follow 

 the fresh combinations produced by the ' reducing division ; ' 

 and the difference between an ordinary nuclear division, and 

 the ' reducing division ' which here takes place, depends 

 essentially on the fact that, in the latter, there is a shifting of 

 the time at which the doubling of the idants occurs. 



Hence, if a species o{ Aiiemia, which had hitherto reproduced 

 bisexually, were now to become parthenogenetic, then in spite 



