THE ORIGIN- AND DEVELOPMENT OF ORGANISMS 331 



when properly nourished, continues to increase in bulk and 

 may grow indefinitely, becoming more and more abundant, 

 but not essentially changing its character. If we admit this 

 power of the germ plasm, the problem of the conformity to 

 type obtains a ready explanation; for some of this germ plasm 

 is simply handed on from one generation to the next, constantly 

 growing in bulk, but not changing its character. At any stage 

 in the development of the race, there is present in each indi- 

 vidual a certain amount of germ plasm, containing in itselt 

 the general race characteristics. The way that this is brought 

 about is believed to be as follows : 



In each egg produced by an animal or a plant, and in each 

 sperm produced by the male, there is a small quantity of this 



FIG. 139. DIAGRAM TO ILLUSTRATE HEREDITY, SHOWING 

 TWO GENERATIONS OF HYDRA 



gp, germ plasm; 

 or, an ovum; 



sp, somaplasm; 

 s, sperm. 



The diagram shows how the germ plasm in the egg, ov, divides: one part, sp, develops 

 into the next generation, while the other part, the germ plasm, gp, becomes stored in its 

 reproductive bodies, ov 1 and s. In 5, the germ plasm from an egg is combining with the germ 

 plasm from the sperm, s 1 , in sexual fertilization. 



germ plasm; Fig. 139 gp. It is the presence of this germ plasm 

 that makes it possible for the egg to develop into a new individ- 

 ual like the parent. An early step in the development of this egg 

 toward the adult consists in the division of this germ substance 

 into two parts. The two are essentially alike and both contain 

 the same characteristics; but each has a different purpose. 

 One of them remains exactly as it is at the start, increasing 



