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 
