6o THE FIRST PRINCIPLES OF HEREDITY 



ship between germ and germ can clearly be traced. Thus, 

 among the Diptera the first segmentation divides the ripe 

 ovum into two cells, of which one is the mother-cell of the 

 future body, while the other is the mother-cell of the germ- 

 cells of the new individual to be formed. In the embryo- 

 genesis of the water-fleas [Daphnides] the primitive germ- 

 cells are separated from the body-cells (" somatic " cells) 

 during the early stages of segmentation, while in the 

 Sagitta-^oxvci this differentiation of the germ-cells takes 

 place somewhat later still during the gastrula stage. In 

 these latter cases the germ-plasm of the next generation is 



B 







Fig. 35. — Development of Sagitta. (After Hertwig.) 

 {From Weismann, "The Germ-Plasm.") 

 A, B, C, three successive stages ; g, germ-cells. 



therefore not immediately separated from the other 

 idioplasm of the body, but is carried along in a latent state 

 in some of the primitive body-cells, during a few of their 

 earlier stages, until it is separated in the form of distinct 

 germ-cells. Now, this differentiation of the germ-cells 

 may be put off until very late in the development of the 

 body structure, the germ-plasm being handed down in a 

 latent, inactive state from cell to cell, without in any way 

 influencing at any stage the body-cell in which it happens 

 to be contained. The route which the germ-plasm 

 follows — i.e., the succession of cells through which it passes 

 until it finally splits off in the form of distinct germ-cells — 



