292 GERM-CELL CYCLE IN ANIMALS 



cells are thus not derived from the individual in 

 which they lie, but have a common origin with it. 

 The segregated germ cells or species substance is 

 therefore distinct and independent of the individual ; 

 this accounts for the constancy of the species. We 

 may distinguish between the two ideas by defining 

 them as follows : 



(1) Germinal continuity, or the germ-plasm 

 theory. "In each ontogeny a part of the specific 

 germ-plasm contained in the parent egg-cell is not 

 used up in the construction of the body of the off- 

 spring, but is reserved unchanged for the formation 

 of the germ cells of the following generation" 

 (Weismann, 1891, p. 170). 



(2) Morphological continuity of the germ cells. 

 The developing egg produces by division two sorts of 

 cells, germ cells which contain the germ-plasm and 

 somatic cells which protect, nourish, and transport 

 the germ cells until they leave the body to give 

 rise to the succeeding generation. 



No case of a complete morphological continuity 

 of germ cells has ever been described. Such an 

 occurrence would necessitate the division of the egg 

 into two cells, one of which would give rise to all 

 of the body cells and nothing else, the other only to 

 germ cells. The behavior of the germ-plasm in such 

 a case would be as follows (Weismann, 1904, p. 410) : 

 "The germ-plasm of the ovum first doubles itself 

 by growth, as the nuclear substance does at every 

 nuclear division, and then divides into two similar 

 halves, one of which, lying in the primordial somatic 



