xxiv Introduction. 



affect the number of men particularly under him, and 

 may alter their original course. 



In cases of alternation of generations, and in those 

 cases in which the two generations do not differ from 

 each other in the full-grown condition, but in which 

 the eggs develop differently, Weismann holds that 

 there must be two kinds of germ-plasm, each containing 

 the determinants of one form, and that these two must 

 be passed separately along the germ-tracks, from one 

 generation to another, so that each must always contain 

 the other, stored away in an inactive condition \ 



In the case of gall-flies there is often a well-marked 

 difference of structure between the females of the two 

 alternating generations. The wing determinants in the 

 winged Trigonaspis crustalis (Plate II, fig. i8 a) must 

 have been modified in the wingless Biorhiza 7'enum, 

 (Plate II, fig. i8), but they appear again in Trigonaspis 

 crustalis. It is improbable that lost determinants 

 would be redeveloped in this way, and therefore there 

 must be two alternating sets of determinants. These, 

 Weismann holds, are contained within the same germ- 

 plasm, but never become active at the same time ^. 



I believe that in the function of the polar bodies we 

 possess the simplest explanation of the problem of 

 alternating determinants in the Cynipidae. 



According to Weismann's earlier views', the first 

 polar body removed ovogenetic nuclear substance which, 

 after the maturation of the tgg, had become superfluous 

 and injurious; while the second polar body removed as 

 many different kinds of idioplasm as were afterwards 

 introduced by the sperm-nucleus. 



He has abandoned this view of the function of the 



^ Weismann, Germ-plasm, p. 176. ^ Ibid. p. 178. 



^ Essays on Heredity ^ vol. i. p. 365, Oxford, 1891. 



