68 The Third and Fourth Generation 



is; it stands, not for a single substance, but for 

 a group of related substances. It is the physical 

 basis of life, that in which life inheres; the living 

 part of animal or plant is always protoplasm. 

 It is by no means invariable; quite the con- 

 trary, it probably has a different composition 

 for each species of animal or plant, perhaps for 

 each kind of tissue. The protoplasm of the cell 

 is differentiated and its parts serve a variety 

 of purposes. The nucleus functions differently 

 from the cell body; the chloroplasts of the 

 plant cell that give it the green color serve to 

 make the sun's energy available for the manu- 

 facture of plant foods. Various foreign sub- 

 stances may be held in the protoplasm and at 

 times be almost indistinguishable from it; such 

 are the food materials in process of elaboration 

 or the waste matters that are on the way to 

 excretion. 



After fertilization, it will be remembered, the 

 egg cell proceeds to divide and subdivide, form- 

 ing a mass of cells that gradually transforms 

 ir^to the embryo and that finally grows, by con- 

 tinued cell division and differentiation, into the 



