BASIS OP THE PKOBLEM 65 



be seen in the living cell. Its detailed structure can, however, 

 only be made out in specimens which have been preserved and 

 stained. The nucleus is then seen to consist of a thin wall within 

 which is contained a colourless sap. In the sap are a number of 

 beads of a darkly staining substance known as chromatin sus- 

 pended on delicate threads of a substance known as linin. The 

 division of a cell is always preceded by the division of the nucleus 

 and, when the nucleus divides, the chromatin undergoes certain 

 remarkable changes. The beads of chromatin become aggregated 

 together into rods the number of rods which appear being 

 invariably constant in the same species, though varying from 

 species to species. These rods are known as chromosomes. Beyond 

 saying that the rods divide into two and that each daughter 

 nucleus, and therefore each daughter cell, is provided with that 

 number of chromosomes which is typical for the species, it is 

 not necessary to follow the details of the process of division any 

 farther. Stated in the briefest possible form, this is what happens 

 during ordinary cell- division, such as that which takes place 

 when a fertilized egg is growing from a single cell into a multi- 

 cellular adult. 



There is one remarkable exception to this type of nuclear 

 division. In the last division but one of those series of divisions 

 which lead to the formation of both male and female gametes, 

 half, and not the full number of chromosomes, is transmitted to 

 each daughter nucleus. If therefore the typical number of chromo- 

 somes in the nucleus of one species is eight and in another four, 

 the number of chromosomes in the gametes will be four and two 

 respectively. In the former species the nucleus of the egg will 

 have four chromosomes, and that of the spermatozoon also four, 

 and the full number typical of the species will only be restored 

 when the nucleus of the spermatozoon fuses with the nucleus of 

 the egg in fertilization. 



The invariable reappearance of the same number of chromo- 

 somes in ordinary cell-division, their reduction to half that number 

 in the last division but one which precedes the formation of the 

 gametes, the complicated mechanism which is employed and 

 other evidence all lead to the conclusion that the location of the 

 basis of the hereditary qualities can be further narrowed and sought 

 in the chromatin in one element, that is to say, of the nucleus. 1 



1 That the basis of the inherited qualities is wholly situated in the chromatin 

 2498 



