5 68 THE POPULAR SCIENCE MONTHLY 



PROTOZOAN GERM PLASM 



By Professor GARY N. CALKINS 



COLUMBIA UNIVERSITY 



IN his classical essays on the nature of the germ plasm, Weismann, 

 more than twenty years ago, drew a distinction between that pro- 

 toplasm destined for the perpetuation of the race and that needed by 

 the organism for its ordinary functions of moving, eating, digesting, etc. 

 The former, which he designated germ plasm, in Metazoa is early dif- 

 ferentiated from the latter, and in some forms may be distinguished as 

 the rudiments of a germinal epithelium even before the end of seg- 

 mentation. The latter develops into the vegetative organs of the adult 

 and serves to nourish and support the former. The distinction, there- 

 fore, especially in the higher Metazoa, indicates a real difference in 

 potential, and the vegetative cells have no primary reproductive func- 

 tions. In lower Metazoa the distinction is not so clear, many of the 

 vegetative cells turning to germ cells either in groups or singly. 



Protozoa, or animals consisting of one cell only, were set apart by 

 Weismann as differing from Metazoa in not showing this somatic 

 germinal differentiation, and he regarded them all as potential germ 

 cells. Furthermore, since germ cells have the possibility, at least, of 

 continued life, while somatic cells die, he assumed that Protozoa are 

 potentially immortal, while natural death is the penalty Metazoa must 

 pay for the privilege of differentiation. 



Weismann's hypothesis is certainly seductive, and, viewed super- 

 ficially, would seem to indicate a fundamental difference between the 

 unicellular and the multicellular animals. Protozoa, however, are 

 more than mere single cells, comparable with the isolated tissue cells of 

 higher animals. They must be regarded as organisms, complete in 

 themselves and comparable, therefore, with the whole animal of higher 

 type and not with any one of its cells. Like the entire Metazoon, it 

 excretes the products of destructive metabolism, it secretes many dif- 

 ferent types of by-products; it moves, obtains food, swallows, digests 

 and assimilates it through the action of digestive fluids; in short, it 

 performs all of the functions which distinguish animals from plants. 

 Finally, like higher types again it reproduces its kind by processes rela- 

 tively as simple as the functions of digestion or nervous response are 

 simple when compared with these functions in Metazoa. In such 

 complete organisms, therefore, it is a logical inference to consider the 

 protoplasm of a protozoon as made up of widely different elements 



