20 



There thus appears to be a mechanism which ensures the 

 presence in every cell in the body of exact representative 

 derivatives of certain bodies the chromosomes that are 

 present in the fertilised ovum, and were there no other com- 

 plications to be faced, it would appear extremely probable 

 that these bodies, and perhaps even particular portions of 

 each of these bodies, represented particular characters. 

 There are, however, other complications which prevent one 

 from accepting this theory. These complications occur in 

 connection with fertilisation, that is, with the production of 

 a new individual. 



We have already seen that at some stage in the life of a 

 multicellular organism a certain group of cells is differen- 

 tiated and destined to be cast off from the body as gametes 

 or sexual elements. As soon as they enter upon this course 

 of differentiation, which is sometimes at quite an early stage 

 in the development of the embryo, these cells live practically 

 as parasites upon the organism in which they occur, and 

 ' cease to form an integral part of it. Like the cells in the 

 soma or body these cells multiply. In the higher animals, 

 however, the ova do not multiply after a very early period 

 in the development of the organism, In sexual cells pro- 

 duced in both the male and female organisms, at a certain 

 stage preceding the production of the mature gametes, a 

 mode of division occurs that is quite different from that 

 described as occurring in the cells that go to build up the 

 body of the animal or plant, and this mode of division 

 introduces that complication to which reference has just 

 been made. In this division the chromosomes appear in 

 half the number that is observed in the cells forming the 

 tissues of the body. The researches of many investigators 

 indicate that this is due to the fact that, during the stage 

 preparatory to division, instead of a spireme being formed, a 

 definite number of separate loops of linin and chromatin 

 appear in the nucleus (Fig. 12). The number of these 

 loops is half that of the chromosomes in the somatic or 

 body cells. Observations also show that these loops are 



