THEORIES OF HEREDITY AND VARIATION. 53 



individual then divides itself into two groups of sixteen cells, each 

 group becoming an individual. After this division a new period 

 of growth begins, and the processes of division are repeated. 



In some forms, after a repeated number of divisions, the cells 

 become weakened or degenerated and are not able to continue the 

 process of growth and division. When this occurs, two cells, or 

 two groups of cells, which have not separated from each other, 

 coalesce or merge into each other and form one stronger individual. 

 After they have remained in this condition for a time they are again 

 able to begin the process of producing new individuals by a series 

 of divisions and subdivisions. In this coalescence we have the 



beginning of sexual propagation. 



I 



DIFFERENT KINDS OF CELLS. 



As we get higher in the scale of multicellular beings, the cells 

 become differentiated, so that to some cells are given certain duties 

 to perform, while to other cells are given certain other duties. In 

 a sense these cells are like the different individuals in civilized 

 societies where one man is a farmer, a second is a tailor, a third is 

 a shoemaker, a fourth is a merchant, and so on. Those cells to 

 which are given the office of reproduction are called germ cells, 

 and the substance of germ cells is called germ plasm. The cells 

 which constitute the body of the individual and form the bones and 

 muscles by means of which the individual is able to move about 

 and secure its food, are called the somatic cells, or simply the soma. 



When we come to man and the higher animals and higher plants, 

 the germ cells are able to propagate themselves, or at least to become 

 multiplied in number, but they are not able to develop beyond the 

 stage of simple cells without coming into contact and merging 

 (coalescing) with cells of a slightly different character, These two 



