14 GENERAL EMBRYOLOGY 



new organisms, although any may do so. But with infrequent 

 exceptions, where unusual methods of reproduction occur at 

 times, a few of which have been noted above, new Metazoan 

 individuals arise only from the union of two germ cells. 



The substance which forms the reproductive cells or gametes of 

 an organism is called the germinal substance, or briefly, the germ. 

 This is visibly distinguishable at a very 'early period in the 

 existence of the new organism, from that material which is to 

 form all of the remainder of the organism, in distinction known 

 as the somatic tissue or soma, or simply as the body. Among 

 the higher forms these two kinds of substance germ and soma 

 have very different histories and fates. According to the 

 theory of Germinal Continuity, elaborated by Weismann, the 

 germ represents or contains an organic substance which has 

 been in a living state since the beginning of life, and which 

 must continue in this state, in some form, as long as living 

 things shall be produced. The soma, on the contrary, is 

 thought to be built up around the germ, anew and under its 

 influence, in each generation of organisms. Upon the death of 

 the individual it is destroyed completely as living substance; 

 somatic cells finally leave no descendants. Thus in species 

 which reproduce by this method the soma or body is wholly 

 temporary, while the germ may properly be said to be poten- 

 tially ever enduring. For while actually the greater part of the 

 germ substance formed in an organism is destined to perish, 

 either before the body or with it, or at any rate with the race, 

 some germ must always remain, producing the generations of 

 the future. The essentials of this idea are expressed in the 

 accompanying diagram (Fig. 11). 



On account of its usefulness the value and significance of the 

 distinction between germ and soma are frequently over- 

 emphasized. In many organisms the distinction can scarcely 

 be drawn at all, for under certain conditions, either normal 

 or unusual, cells which are evidently " somatic " may take 

 on reproductive characteristics and function as germ cells. 

 Many such cases are known among animals, and among the 

 plants reproduction from somatic tissues and cells is very 



