The Germ Cells 251 



detached from the body of the parents. It is in these germ 

 cells or gametes that we should seek the mechanism that 

 brings about the continuity of species. The progeny is like 

 the parents because of something in the germ cell. We have 

 already seen (page 124) that the fertilized t^^ divides into 

 two cells and that each of these divides again in the initial 

 stages of the individual's development. Cell division is a 

 normal process found in all kinds of plant and animal cells. 



Careful study of the cell structure at the time of di- 

 vision shows peculiar changes taking place in the kernel or 

 nucleus of the cell. The nucleus consists of several different 

 substances which can be discerned under the microscope 

 after suitable treatment with various chemicals and stains. 

 Just before the division there appears in the nucleus of the 

 cell a tangle of substance called chromatin, which divides 

 up into a definite number of pieces called chromosomes. 

 The number of chromosomes is the same in all the cells of 

 the body, but it is not the same in all species of plants and 

 animals. The fruit fly, for example, has four pairs or eight 

 distinct chromosomes. The cells of a wheat plant show eight 

 pairs or sixteen chromosomes. In man, potatoes, and onions 

 the number is twenty-four pairs or forty-eight chromosomes. 



During the process of cell-division each chromosome 

 splits lengthwise and the two halves move to the opposite 

 poles of the nucleus (see Fig. 66) . The two groups of chro- 

 mosomes assemble and the several pieces in each gradually 

 merge together into a new nucleus. A separating partition 

 is often formed between the nuclei and thus two new cells are 

 established. This is in general the procedure in all kinds of 

 plant and animal cells. 



A suggestion had long ago been made that the compli- 

 cated movements of the chromosomes in preparation for cell 

 division and the precise separation of each chromosome into 

 two identical halves must have some significance in trans- 

 mitting the organism's characters from cell to cell in the 

 formation of the body, and eventually from parent to off- 

 spring in the formation of new germ cells. The confirma- 



