The Evolution Theory, Vol. 1 of 2

by August Weismann

The beginning of a so-called 'spore-formation' is to be found in many Infusorians. Thus the holotrichous species, Holophrya multifiliis (Fig. 61), reproduces by first becoming enclosed in a cyst or capsule, and then dividing many times in rapid succession, so that 2, 4, 8, 16, &c. individuals arise consecutively, and subsequently burst forth from the cyst (Fig. 61, B). In the Gregarines and other Sporozoa the period of division lasts much longer, and the encysted animal divides into 128, 256, or even more portions; but in this case also each part or 'spore' receives a piece of the maternal cell-body and cell-nucleus, so that there is no difference in principle between this and the simple division into two exhibited by Stentor; as in that case, so here, it is not the fully differentiated structure of the animal which is handed on to the divided parts; it is only the power to redevelop this anew on their own account. Thus here again we are face to face with the fundamental problem of heredity: How is it possible that the power of reproducing the complex whole can be inherent in the simple parts?

Fig. 61. Holophrya multifiliis, an Infusorian parasitic on the skin of fishes. A, in its usual condition; ma, macronucleus; mi, micronucleus; cv, contractile vacuole; m, mouth. B, after binary fission has been several times repeated within the cyst (cy); tt, results of the division. C, one of these units much enlarged.

In contrast to the unicellular organisms, the great majority of the multicellulars, the Metazoa and Metaphyta, many-celled animals and plants, differ not only in the multitude of their cells, but even more in the manifold differentiation of these cells according to the principle of division of labour, so that the various functions of the animal are not performed by all the cells uniformly, but each function is relegated to a particular set of cells specially organized with reference to it. Thus there is differentiation between motile, nutritive, and reproductive cells, and there may also be glandular, nerve, muscle, and skin cells, and we know how this differentiation into a great number of different kinds of cells with highly specialized functions has arisen, especially among the higher animals, in a multiplicity which cannot easily be overlooked. Thus we find a large number of the most diverse kinds of cells, all of which serve for the maintenance of the body, in contrast to the simply reproductive cells or germ-cells. These alone possess the power of reproducing, under certain conditions, a new individual of the same species. We can contrast with these germ-cells, which serve, not for the maintenance of the individual, but only for that of the species, all the other kinds of cells under the name of somatic or body-cells. The problem which we have to solve now lies before us in the question, How comes it that the germ-cell is able to bring forth from itself all the other cells in definite sequence and arrangement, and is thus able to build up the body of a new individual?

The similarity of this problem to that formulated in regard to unicellular organisms is at once obvious, but it becomes still more emphatic when we remember that the gulf between unicellular organisms and the higher animals and plants is bridged over by certain transition forms which are of the greatest interest, especially in relation to the problems of inheritance.

Fig. 62. Pandorina morum; after Pringsheim. I, A young colony, consisting of 16 cells. II, Another colony, whose cells have reproduced daughter-colonies; all the cells uniformly alike. III, A young Volvox-colony; sz, somatic cells; kz, germ-cells.

Among the lower Algæ there is a family, the Volvocineæ, in which the differentiation of the many-celled body on the principle of division of labour has just set in; in some genera it has been actually effected, though in the simplest way imaginable, and in others it has not yet begun. Thus in the genus Pandorina the individual consists of sixteen green cells, united into a ball (Fig. 62, I), each one exactly like the other, and all functioning alike. They are all united into a spherical body, a whole, by a gelatinous matrix which they all secrete, and thus they form a cell-colony, a cell-stock, a many-celled individual; but each of these cells has not only all the typical parts—cell-body, nucleus, and contractile vacuole—but each possesses a pair of flagella or motor organs, an eye-spot, and a chlorophyll body which enables them to assimilate nourishment from the water and the air. Each one of these cells thus performs all the somatic functions, that is, all that are necessary to the maintenance of the individual life. But each also possesses the power of reproducing the whole colony from itself, that is, it also performs the function of reproduction necessary to the maintenance of the species. When such a colony, whose sixteen cells are continually growing, has led for some time a free-swimming life in the water, the cells retract their flagella, and each begins to multiply by dividing into 2, 4, 8, finally into 16 cells of the same kind, which remain together, forming a spherical mass enclosed in a gelatinous secretion (Fig. 62, II). Thus there are now, instead of sixteen cells in the mother-colony, sixteen daughter-colonies, each with sixteen cells which soon acquire flagella and eye-spots, and are then ready to burst forth from the dissolving jelly of the maternal stock as independent individuals. This Pandorina shows no trace of a differentiation of its component cells to particular and different functions, but a nearly allied genus of the same family, the genus Volvox (Fig. 62, III), consists of two kinds of cells—on the one hand of small cells (sz) which occur in large numbers and compose the wall of the hollow gelatinous mass, forming, so to speak, the skeleton of the Volvox; and, on the other hand, of a much smaller number of cells which are very much larger (kz). The former, the 'body' or 'somatic' cells, are green, and have a red 'eye-spot' and two flagella; they are connected with each other by processes from their cell-bodies, and are able, by means of the co-ordinated action of their flagella, to propel the whole colony with a slow rotatory movement through the water. Many of my readers are doubtless familiar with these light green spheres, which are quite recognizable with the naked eye, and people our marsh pools and ponds in Spring in such abundance that it is only necessary to draw a glass of water to procure a large number of them.

The little flagellated cells just described serve not only for the locomotion of the colony, but also for nutrition, for the secretion of the jelly, and for the excretion of waste products; in short, they perform all the functions necessary to the maintenance of life, but not that of reproduction. They can, indeed, multiply by dividing when the colony is young, like the cells of Pandorina, but they cannot reproduce the whole colony but only cells like themselves, that is, other somatic cells. In Volvox the maintenance of the species, the production of a daughter-colony, is the function of the second and larger kind of cells, the reproductive cells, which are contained in the interior (filled with a watery fluid) of the gelatinous sphere. They possess no flagella (kz), and so take no share in the swimming movements of the somatic cells. For the present we need not allude to the fact that there are several kinds of these cells, and need only state that the simplest among them, the so-called 'Parthenogonidia,' after they have reached a considerable size, begin a process of division which results in the formation of a daughter-colony. Usually there are several of these large reproductive cells in a Volvox colony, and as soon as these have developed into a similar number of daughter-colonies they burst out through a rupture in the now flaccid jelly of the maternal sphere and begin to lead an independent life. The mother-sphere, which now consists only of somatic cells, is unable to produce new reproductive cells; it gradually loses its spherical form, sinks to the ground, and dies.

In Volvox we have, for the first time, a cell-colony in which a distinction has been established between body or somatic cells and reproductive or germ-cells. In contrast to Pandorina, a large number, indeed the majority of the cells of the colony, have lost the power of reproducing the whole by division, and only the few reproductive cells possess this, while they, in turn, have lost other functions, notably that of locomotion. Their power of reproducing the whole, that is to say, their hereditary capacity, gives them a greater theoretical interest than the cells of Pandorina, for the latter require only to produce others like themselves, because there is only one kind of cell in the colony, while in Volvox the reproductive cell can not only produce others like itself, by division, but can produce the body-cells as well. The problem is quite analogous to the one which we have had to face in regard to the unicellular animals of complex structure, the Infusorians. The question, How can the part of the trumpet-animalcule which is mouthless develop from itself a new mouth and ciliated apparatus? here transforms itself into the question, How can a cell by division give rise not only to others like itself, but also to the body-cells, which are of quite different structure? This is, in its simplest form, the fundamental problem of all reproduction through germ-cells, to which we must now pass on. But first a short digression.