16 GENERAL EMBRYOLOGY 



only at irregular intervals that the individuals ordinarily 

 multiplying by fission, lose this property as well as their 

 vegetative characteristics, become specialized as gametes, and 

 require to undergo syngamy, later resuming their duplex 

 vegetative and reproductive character. 



While it might be misleading to say that the reproductive 

 cells of Metazoa are, like the Protozoa, both germ and soma, yet 

 it is quite true that in these germ cells we have a substance 

 which produces both germinal and somatic tissues. In a 

 sense we are hardly justified in saying that the soma is built up 

 anew in each generation, while only the germ has a continuous 

 existence. ^The germ cell is potentially soma as well as germ, 

 and for a time during the early development of the organism 

 there is no visible distinction; this distinction occurs very early 

 in the development of a few forms, but in most organisms not 

 until a considerable number of cells has been formed. In 

 development the germ cells give rise to other cells like them- 

 selves (germ) and to cells unlike themselves (soma) and we 

 may regard the " unlike" as "new." 



The common conception of the life of a species as a succession 

 ;of generations of individuals linked together by the germ, 

 /while superficially true, leads to a fundamentally erroneous 

 / point of view. The fertilized germ cell is just as much the 

 individual organism as the matured individual is. The species 

 is no more a succession of somas than it is a continuous germ. 

 It is not the function of the germ to provide links between 

 successive generations of "organisms" or somas, any more than 

 it is the function of the soma to insure the continuity of the 

 germ, and to provide materials for its increase and means of its 

 dispersal. We should recognize that the essential continuity 

 between successive generations is, after all, not continuity of 

 plasm but "continuity of organization." 



The term reproduction, strictly speaking, does not mean 

 quite the same thing among Metazoa and simple Protozoa. 

 Among the Protozoa the formation of free daughter cells, by 

 fissions of the zygote or its descendants, constitutes repro- 

 duction. Among the Metazoa the corresponding fissions of the 



