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 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.) no attempt has been made even to indicate the many variations in detail which occur in different animals and plants. These are numerous, but do not affect the general plan nor the fundamental goal, which is always and invariably the same-viz., the equal, longitudinal diviIsion of the chromosomes or the chromatin band of the parent nucleus between the two daughter nuclei. "" Direct division. Another method of cell division, formerly taken to be the universal one, has since been shown to occur solely as a stage in the degenerative changes of cells which are upon the downward road to disintegration, and in which the power of multiplication is about at an end. In this, the "direct" or "amitotic form of cell division, the cell nucleus is simply constricted into two portions preceding the constriction of the cytoplasm. This method stands in marked contrast to the elaborate mechanism which insures the exact distribution of the nuclear substance in the karyokinetic or indirect method. Every multicellular organism arises by a process of division from a single cell, the fertilized germ or egg cell, which in turn has been cut off from the cells of a pre-existing individual. Out of the group of cells which result from the continued division of the germ cell and its descendants are differentiated the various tissues and organs of the body through which the vital functions are carried on. Those tissues and organs which perform functions pertaining directly to the existence of the indi 64 Somatic" and reproductive tissues. vidual have been termed "somatic," and their constituent cells the "somatic or body cells, in contradistinction to the reproductive tissues or cells whose function concerns the continuance of the species. In some forms these two groups of cells, the somatic and the reproductive, become isolated from each other quite. early in development; in one case, indeed, the differentiation of reproductive cells from the somatic ones has been traced by Boveri back to the first division of the egg. This case of Ascaris megalocephala is so striking and of such fundamental theoretical importance that it must not be passed without notice, for in it we find marked differences between the somatic and reproductive cells in their nuclear structure, their relative amount of chromatin, and mode of division. The egg of Ascaris has been the classical object for cytological studies on account of its small number of chromosomes (two in variety univalens, four in bivalens), their large size, and the diagrammatic clearness of the changes which take place in division. In the division of the fertilized egg cell we have two (in univalens) long chromosomes handed over to each daughter cell. As these two cells in Differentiation of turn divide, a striking difference is seen reproductive and in the karyokinetic figures. In Fig. 6, somatic tissues A, such a two-celled stage is seen from in Ascaris. the pole; in Fig. 6, B, a slightly later stage in side view of the spindle. In the upper cell of Fig. 6, A, the division is of the usual form, the two chromosomes split longitudinally, and their two halves. travel to opposite poles of the spindle (Fig. 6, B). But in the lower cell this is not the case. The central portion of the two chromosomes is broken up into a large number of minute chromatin granules which divide, and, as shown in Fig. 6, B, form the only portion of the chromosomes drawn up to the poles and entering into the structure of the resting nuclei after the division is complete. The large swollen outer ends of the chromosomes are cast off into the cytoplasm and are eventually absorbed, playing no further part as nuclear structures. Fig. 6, C, shows the four-celled stage, in which a marked |