4 HEREDITARY CHARACTERS 



" gametophytes," which are in reality separate individuals 

 growing upon the plant. Strictly speaking, it is not the 

 plant itself which must be regarded as corresponding to 

 the body of the animal, but the gametophyte. In the 

 higher plants, therefore, as in the higher animals, two 

 individuals may be said to be usually necessary to the 

 production of a new individual. 



All multicellular organisms commence their existence 

 as single cells. This single cell divides .into two cells. Each 

 of the two thus produced divides again, and this process 

 continues until the whole body of the multicellular ani- 

 mal or plant is built up. Among the cells of the earlier 

 generations there is a great similarity in appearance ; in 

 fact, until a great number of cells have been produced, it 

 is impossible to see any differences between them. As 

 more and more are produced, however, they become divided 

 into groups, and the individuals forming each of these 

 groups are often very different indeed from those forming 

 others. This differentiation appears very gradually, and it 

 is only when the organism is well advanced in its develop- 

 ment that the complete differentiation between various kinds 

 of cells is reached. It is due to the fact that the cells pro- 

 duced from the original one develop along different lines, 

 that the tissues of the body differ from each other. The 

 final stage of differentiation in the enormous number of 

 cells produced from the single original cell forms the com- 

 plete body of the multicellular organism. One group of 

 cells, however, does not, in the case of many organisms at 

 any rate, act in this manner. The cells composing it do not 

 become modified in shape and function as do those belong- 

 ing to other groups. Instead of serving some purpose 

 necessary to the well-being of the organism, as is the case 

 with all the rest of the cells, those belonging to this group 

 are destined to be thrown off from the body, and to go to 

 the formation of new individuals. Two of these cells thrown 

 off from different individuals (in the case of the higher 

 animals and plants usually from individuals of opposite 



