FIG. 1.-DIAGRAM OF KARYOKINESIS (AFTER WILSON).
(By permission of MR. EDWARD ARNOLD.)

A, Cell in state of rest (csm = centrosome). B, chromatin (chr) coiled in chain-like shape. Centrosome halved and surrounded by aster. C, spindle formed, nuclear membrane dissolving (sp=spindle; chr=chromosomes now distinct). D, karyokinetic figure (csph= centrospheres, forming poles). F, chromosomes definitely halved, aggregated at the poles. G, the new nuclei, after completion of karyokinesis (ns = nucleus; csm=centrosome).

of the parent, is the chromatic substance, or, as Weismann has called it, the Keimplasma or germ-plasm. It is always the vehicle of the hereditary qualities, no matter what the precise mode of reproduction may be.

It may be remarked that the nuclei of the egg-cell and of the sperm-cell have equal biogenetic value. It is a mistake to speak of the egg-cell as female, and of the sperm-cell as male. Each of these germ-cells has a complete equipment of chromosomes, and each may contain the centrosome which plays such an important part in the complicated process of karyokinesis. As a rule, however, it is the sperm-cell which imports its centrosome into the ovum. There is, however, no opposition or difference between the properties of the two cells, and the nucleus of the sperm-cell can take upon itself the rôle of the egg nucleus and vice versa. This fact has been demonstrated by Boveri's experiment on the egg of the sea-urchin. Boveri succeeded in separating the nucleus of an egg-cell from the surrounding protoplasm; he then artificially fecundated the denucleated egg-cell with the sperm of a male sea-urchin. The fertilised ovum-with a sperm nucleus only-proceeded to develop, the embryonic process pursued a normal course, and a larva was produced, capable of swimming freely. Weismann has concluded from this experiment that the nuclei of the two kinds of germ-cells are in every respect equivalent, and that each is complete in itself.

Before proceeding to consider the germ-plasm in more detail, we must glance at certain other phenomena presented by the process of fertilisation. It has now been ascertained that the phenomenon of fertilisation is essentially bound up with the blending of two nuclei. Fertilisation may, indeed, be described as the mingling of nuclei (amphimixis). A good example of such mixing is seen in the conjugation of unicellular organisms, in which it often happens that two individuals become one, just as the two germinal individualities or gametes-ovum and spermatozoon-in the Metazoa become very intimately one in ordinary

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fertilisation. In both cases the process is a decrease in the number of individualities, as a preliminary to subsequent in

crease.

Professor Ischikawa, of Tokio, has given a very thorough description of the phenomenon of conjugation in the phosphorescent flagellate Protozoa known as Noctiluca. Two pinhead-like individuals unite for the purpose of conjugation. The protoplasm of the two cells is mingled; the nuclei, although not altogether blended one with another, form a single karyokinetic figure; a karyokinesis sets in, in the course of which the chromatic substance of the two conjugates is divided equally between the nuclei of the two daughter organisms. Finally, as a result of the karyokinesis, two distinct Noctilucas are formed. We see, therefore, that, although the phenomenon of conjugation results for a time in the diminution of the number of individuals, the ultimate result is their multiplication.

A remarkable phenomenon presented by all multicellular organisms, from the lowest to the highest, is the maturation of the germ-cells, which is accompanied by a reduction of the number of chromosomes to one-half of their original number. In the case of the egg-cell, the original germ-cell, through growth and the doubling of the number of its chromosomes, gives rise to the "ovocyte," or immature egg-cell (Fig. 2, B). This ovocyte goes through two successive divisions. The result of the first division is to divide the number of chromosomes into equal halves. Thus, if we take four chromosomes as the normal number for a given species, and suppose that number to have been doubled in the course of the development of the original germ-cell, we find that four chromosomes are, as a result of this first division, ejected from the cell, so to speak. These four chromosomes form the first "polar body," lying against the exterior wall of the cell (Fig. 2, D, pg). The remaining half of the chromosomes lie juxtaposed to the polar body, but within the cell, forming a reduced nucleus. By a second division this

nucleus is again halved, two additional chromosomes being ejected, and forming the second polar body (E). Meanwhile the first polar body divides into two, each with two chromosomes. Thus there are eventually four nuclei produced from the original

FIG. 2.-MATURATION OF OVUM OR EGG-CELL (AFTER WEISMANN). (By permission of MR. EDWARD ARNOLD.)

A, The primitive germ-cell (chr = chromosomes). B, the ovocyte, with doubled chromosomes. C, first karyokinesis. D, immediately afterwards, half the chromosomes (4) in the first polar body (pg). E, second karyokinesis, first polar body (pg) divided, nucleus effecting new division. F, the mature ovum, with two chromosomes; three polar bodies outside the cell.

ovocyte-namely, three polar bodies and the reduced nucleus of the ovum-and each of the four nuclei has two chromosomes -half the normal number.

The chief interest of this complicated maturation process

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centres in the chromosomes. The doubling of their number during the development of the primitive germ-cell is very remarkable. The reason for it may be found, as Weismann has suggested, in the fact that it enables the number of variations produced by amphimixis, or the mingling of the parental elements, to be enormously increased. The successive divisions preceding maturation would, in the event of the number of chromosomes not being doubled, have the effect of reducing the chromosomes to one-fourth of the normal number possessed by the species. As it is, the possible combinations of the chromosomes are very greatly increased through the doubling which precedes their ultimate reduction; for it must be observed that the next result of maturation, as far as the chromosomes are concerned, is their reduction by half. Thus, in the hypothetical case before us, the number of chromosomes in the original germ-cell is four; after the maturation the ovum contains but two (Fig. 2, F).

As for the polar bodies which are "expelled," so to speak, from the egg-cell during its maturation, there seems little doubt that they are abortive ova, incapable of attracting the sperm-cell, and destined only to disappear. The difference between the maturation of the sperm-cell and that of the ovum is that, whereas the four spermatozoa which result from the successive divisions of the spermatocyte (corresponding to the ovocyte) remain complete, and potentially capable of fertilising the egg, three of the products of the ovocyte are eliminated in the form of polar bodies, and only one-the mature ovum-is capable of development. The reason for the impotency and essential disappearance of the polar bodies is to be found in the fact that they lack the protoplasmic substance necessary to maintain life. The process of maturation in the case of the spermatozoon is much the same as in that of the ovum. The primitive germ-cell goes through a period of growth, and develops into the spermatocyte. During this development the number of chromosomes is doubled. The spermatocyte then undergoes two successive