FERTILISATION 443 



loom " (Wilsoiij i8g6, p. 171). While the ovum-centrosome 

 of many animals seems to disappear, that introduced by the 

 spermatozoon divides into two, and around each a system of 

 rays develops. The sperm-centrosomes migrate to opposite 

 sides of the segmentation nucleus, and between them appears 

 the spindle of the first cleavage. It may be hasty to call 

 them " kinetic centres," but they seem to have an important 

 role in the division-process. 



Let us suppose that a young egg-cell has sixteen chromosomes or 



idants, 16A ; 

 in the course of maturation the number is reduced by a half to 



8A; 

 the mature egg-cell is fertilised by a (reduced) spermatozoon with 



eight chromosomes, 8B ; 

 the fertilised egg-cell has then eight maternal and eight paternal 



chromosomes, 8 A -|- SB ; 

 the young germ-cell capable of initiating a ; n generation has the 



same ; 

 in the maturation of this young egg-cell reduction occurs to 4A 



+ 4B; 

 it is fertilised by a sperm of analogous history with 4C -\- 4D ; 

 the fertilised egg of the second generation has therefore 4A -(- 4B 



+ 4C + 4D ; 

 similarly, the fertilised egg of the third generation may have 2A 



+ 2B + 2C + 2D -{- 2E + 2F 4- 2G + 2H ; 

 similarly, in the fourth generation the chromosomes may be 



A4-B4-C4-D4-E4-F-t-G4-H4- I + j4-K + L + M-f 



N -(- O + P (sixteen all different). 



But the number of different chrornosomes need not mount up so 

 rapidly, for some of the paternal chromosomes may be the same as 

 maternal. Moreover, the reducing division need not leave the 

 maximum number of different chromosomes. The number sixteen, 

 by hypothesis characteristic of the species, cannot be exceeded ; 

 but the heterogeneity may spread into the individual chromosomes, 

 affecting the ids. 



Summary. — Put as simply as possible, the case is as follows. 

 The independenth' heritable and variable qualities of an 



