66 THE FIRST PRINCIPLES OF HEREDITY 



the effect of removing each time half the number of chromo- 

 somes from the germ-cell, so that by the union of two germ- 

 cells in the act of fertilization the original normal number 

 is once more restored. 



The reducing division has the further effect of changing 

 the composition of the germ-plasm of each germ-cell. If we 

 assume that before the introduction of Amphimixis among 

 organisms (see Fig. 38) the germ-plasm of the male germ- 

 cell had 16 Idants (16A), all equal, we should get, after the 

 reducing division, 8A, and if these combine with SB from 



a female germ-cell, we get a 

 germ-plasm, 8A-f8B. Now, 

 a further reducing division 

 takes place in the next 

 generation, and leaves for the 

 germ-cell again half the num- 

 ber — i.e., 4A-I-4B. These, 

 combined with 4C + 4D from 

 the mother, give us a germ- 

 ^ plasm of the composition 

 4A + 4B-1-4C-1-4D. If we re- 

 duce again by half, and so 

 on, we obtain ultimately a 

 germ-plasm consisting of 16 

 different Idants, A, B, C, 

 D, E, F, etc., though in reality this reducing division may 

 not have taken place so regularly. 



But this process goes farther still. Not only all the 

 Idants become individually different, but also their Ids. 



We have already pointed out in a previous chapter that 

 the reducing division differs from the ordinary cell-division 

 in that it divides the chromatin into two qualitatively 

 different parts. We must now add that this division does 

 not necessarily fall between the individual chromosomes 

 or Idants. If we have (see Fig. 39) 4 Idants, A, B, C, D, 

 each with 6 identical Ids — i.e., Idant A with 6 Ids a ; 

 B with 6 Ids h ; C with 6 Ids c ; and D with 6 Ids d — then, 



Fig. 39. — Change of Ids 

 (After Delage.) 



