cells are the remnants of the ends of the chromosomes which have been cast off in the division. In Fig. 6, D, the four-celled stage is shown with the karyokinetic figures of the next division. In the lower cells the spindles are seen from the pole, the chromatin is present in the reduced amount in the form of small granules. In the upper left-hand cell the two full chromosomes are seen, each split longitudinally, while the upper righthand cell shows a repetition of the reduction phenomenon -viz., the central portion of the two chromosomes, broken up into granules, alone enters into the spindle figure, the outer ends being cast off into the cytoplasm, where they suffer a similar fate to those of the lower cell in the previous division. The next division repeats the process, one cell retaining two full chromosomes, while all the others have the reduced amount. This takes place for five successive divisions and then ceases; from the one cell having the two full chromosomes, the reproductive tissues develop, the others with reduced chromatin form the somatic tissues. Thus is accomplished a visible structural differentiation of the nuclei of the reproductive cells which distinguishes them sharply from all the somatic tissues in Ascaris. We shall see further on that there is abundant evidence in favour of the theory that the nucleus-i. e., the chromatin-is the bearer of hereditary influences from one generation to the next, and that the specific development and functions of each individual cell are dependent upon the specific changes which take place in the chromatin of its nucleus. In this light the almost isolated case of Ascaris possesses a value and interest that can not be overestimated. While in the higher forms of animals and plants we find a sharp differentiation of their tissues into somatic and reproductive or germ cells, we must bear in mind that not in all forms is this power of the reproduction of the whole organism so sharply limited to the germ cells alone. The familiar propagation of plants by cuttings, the regeneration of complete animals from small portions of their somatic tissues in many lower forms, and numerous other considerations such as these, show clearly that the difference between the powers of somatic and germinal cells is but one of degree; that while in higher organisms the two seem sharply defined from each other, a series of lower forms may be taken which will show the intermediate steps in this gradual specialization of function. Reproduction in In the unicellular organisms we have most interesting examples of the fundamental facts of reproduction, and through an examination of these we may gain an insight into the more complicated processes of the Metazoa. Each of these lowest forms consists of a single cell in which are carried out in a generalized way the complex physiological functions which, in many celled animals, are divided up among cell groups. In reproduction the animal simply divides into two, the division of the nucleus preceding that of the cytoplasm, and the method is usually a more or less modified karyokinetic one. This mode of multiplication continues in most forms for a certain number of generations, and then the necessity for conjugation-i. e., a temporary or permanent fusion with another individual—sets in. If this conjugation be prevented, the animal Conjugation. soon shows increasing signs of degeneration which result in death. This " senescence" of the powers of growth and multiplication can only be checked by the admixture of new nuclear substances from an entirely different individual by conjugation. In its simplest terms this process is found in Chilodon, according to Henneguy. Chilodon is a minute freshwater infusorian, which multiplies for a considerable period of time by transverse division. After a time, however, the physiological necessity for conjugation. ensues. The animals having placed themselves side by side in pairs and partly fused together, the nucleus of each individual divides into two portions, one of which passes from each infusor into the other to unite with the half remaining stationary. The two then separate, each having received a half of the nucleus of the other. After thus trading experiences, as it might be termed, a period of renewed vigour and activity for each sets in, manifested in rapid growth and multiplication by division, producing a large number of generations, which continues until weakening vital activities indicate the periodically recurring necessity for conjugation. In general, among the Infusoria we find the same process taking place in regular cyclical order, with more or less. complicated variations of the phenomena just outlined for Chilodon. In all of them the aim of the conjugation is the same, the exchange of a certain amount of nuclear substance between the two conjugating individuals, and the same physiological effect is reached, a rejuvenescence, as it were, of the two organisms which manifests itself in renewed vigour of growth and multiplication. In some of the lowest forms of unicellular life—for example, the Schizomycetes or bacteria and their allies -this necessity for conjugation does not appear to exist, but for the vast majority of forms this cyclical law of development holds good. In the Protozoa no division into somatic and germinal cells is found, both functions being united in the one cell which forms the whole body of the organism. In the Metazoa, however, this differentiation has taken place; the germinal cells are set apart for the preservation of the race; the so Gradual differentiation of reductive cells. matic cells carry on their various functions for a time, grow old, die, and disappear, certain of the germ cells alone surviving in the production of new individuals. On the borderland between the unicellular and the multicellular organisms, however, stand certain colonial forms, which show an exquisitely graded series of steps, from the conditions of unicellular multiplication to those of the multicellular forms. Let us examine a few examples of these. Pandorina morum is a minute fresh-water Alga, consisting of a colony of sixteen ovoid cells imbedded in a spherical mass of a jelly-like substance. From each of these cells two long, hairlike flagellæ extend out freely into the water, and by their lashing to and fro the colony is propelled from place to place (Fig. 7, A). In multiplication by simple division each one of these cells divides into a group of sixteen daughter cells, the general gelatinous intercellular substance of the parent colony dissolves, the sixteen daughter colonies become free, and by continuous growth soon attain the size of the parent colony (Fig. 7, B). After a certain number of generations produced in this manner, the necessity for reproduction by conjugation ensues. In this method the sixteen cells of a colony divide, each one usually into eight minute cells, which are set free in the water by the dissolution of the common gelatinous envelope (Fig. 7, C). Each one of these swarm spores, or zoospores," consists of an oval, greenish cell, the pointed end of which is hyaline and bears two long cilia, by means of which the spore swims through the water (Fig. 7, K). These zoospores are not all of exactly the same size, but no great difference is noticeable. If the zoospores from two different colonies come near each other, they unite in pairs made up of individuals of the same or of different 66 sizes (Fig. 7, D). These coalesce, round up into a spherical cell (Fig. 7, E, F), which soon develops an enveloping cellulose wall, and passes as a "zygote" into a resting stage (Fig. 7, G). In this condition the organism FIG. 7.-Development of Pandorina morum: A, a swarming family: B, a similar family divided into sixteen daughter families; C, a sexual family, the individual cells of which are escaping from the common gelatinous investment; D, E, conjugation of pairs of swarm spores; F, a young zygote; G, a mature zygote; H, transformation of the contents of a zygote into a large swarm cell; I, the same, free; J, a young family developed from the latter; K, a free swimming swarm spore, (After Pringsheim.) |