Page:Popular Science Monthly Volume 79.djvu/572

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N his classical essays on the nature of the germ plasm, Weismann, more than twenty years ago, drew a distinction between that protoplasm 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 differentiated from the latter, and in some forms may be distinguished as the rudiments of a germinal epithelium even before the end of segmentation. The latter develops into the vegetative organs of the adult and serves to nourish and support the former. The distinction, therefore, especially in the higher Metazoa, indicates a real difference in potential, and the vegetative cells have no primary reproductive functions. 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 superficially, 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 different 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 relatively 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