in definite sequence and arrangement, and is thus able to build up the body of a new individual? [Illustration: 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.] 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. 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