It is necessary to explain here the results of recent scientific work on the intimate phenomena of cell-division, for they are closely allied to those of fecundation.
The nucleus of an ordinary cell presents itself in the form of a nearly spherical vesicle. Delicate methods of staining have shown that the nucleus encloses several round nucleolar corpuscles, and also a reticulum which is attached to its membrane and spreads through its whole substance. The liquid part of the nucleus fills the meshes of this reticular tissue, which stains easily and for this reason is named chromatin. The phenomena of cell division in well-developed cells with nuclei is termed mitosis. Certain lower forms of cells exist in which the nucleus is not well differentiated. Mitosis begins in the nucleus (Plate I). Figure 1 represents the cell before division has commenced. In the protoplasm, by the side of the nucleus, is formed a small corpuscle (c) which is called the centrosome. The nucleus itself is marked b. When the cell commences to divide, the meshes of the network of chromatin contract and the centrosome divides into two parts (Fig. 2). Shortly afterward the particles of chromatin concentrate in the form of convoluted rods called chromosomes (Figs. 3 and 4). The number of these varies according to the species of organism, but remains constant for each animal or vegetable species. At the same time the two centrosomes separate from each other on each side of the nucleus. The chromosomes then become shorter and thicker while the nucleus is completely dissolved in the protoplasm of the cell, and its membrane disappears (Fig. 4).
Directly afterwards the chromosomes arrange themselves regularly in line, like soldiers at drill, following one of the larger diameters of the cell, and forming a barrier between the two centrosomes (Fig. 5). Each of the chromosomes then divides into two parallel halves of equal thickness (Fig. 6).
Figures 3 and 4 show that, while these changes are being produced, each of the two centrosomes is surrounded by stellate rays. Some of these rays extending in the direction of the chromosomes, become attached to one of their extremities and draw it toward the corresponding centrosome (Fig. 7). Thus around each centrosome are grouped as many chromosomes as the mother cell possessed itself (Fig. 8). Simultaneously, the cell enlarges and its protoplasm commences to become indented at each end of the diameter previously formed by the chromosomes. From this moment the nuclear liquid concentrates itself around each of the groups of chromosomes, the rays disappear and the cell divides into two halves, each containing a group of chromosomes (Fig. 9); the indentation increases so as to form a partition across the protoplasm. The chromosomes then form a new meshwork of nuclear chromatin, and we have then two cells each with a nucleus and a centrosome like the mother cell (Fig. 10).
This is what takes place in the reproduction of all cells of the animal and vegetable kingdoms. In the simplest unicellular organisms which are known fission constitutes the only means of reproduction. In the complicated organisms of the higher plants and animals each cell divides in the manner indicated above, both in the embryonic period and later on during the development of each of the organs which forms the organism. This fact shows more than any other the intimate relationship which connects all living organisms. The most remarkable thing, perhaps, is the almost mathematical division of the chromosomes into two halves, a division which results in the equal distribution of their substance through the whole organism. We shall return to this point later on.
Reproduction by Budding. Parthenogenesis. In the animal and vegetable kingdoms the higher organisms become more and more complicated. They are no longer composed of a single cell, but of an increasing number of these cells combined in a whole, of which each part, adapted for a special purpose, is itself formed of cells, differentiated as much by their organic form as by their chemical and physical constitution. In this way, in plants, are formed the leaves, flowers, buds, branches, trunk, bark, etc.; and in animals the skin, intestine, glands, blood, muscles, nerves, brain, sense organs, etc. In spite of the great complication of the divers living multicellular organisms, one often finds among them the power of reproduction by fission or by budding. In certain animals and plants, groups of cells vegetate in buds which separate from the body later on and form a new individual; this occurs among the polypi and plants with bulbs, etc. One can even form a tree by means of a cutting. Ants and bees, which have not been fecundated, are capable of laying eggs which develop by parthenogenesis (virgin parturition) and become complete individuals. But these degenerate and disappear if reproduction by parthenogenesis or budding is continued during several generations.
Among the higher animals, the vertebrates and man, there is no reproduction without conjugation; no parthenogenesis or budding. So far as we have studied the question we see in the animal and vegetable kingdoms sexual reproduction, or conjugation, as a sine qua non for the indefinite continuation of life.
The Sexual Glands. The Embryo. However complicated the organism, it always possesses a special organ, the cells of which, all of the same form, are reserved for the reproduction of the species and especially for conjugation. The cells of these organs, called sexual glands, have the power of reproducing themselves so that they reconstruct the whole individual (the type of the species) from which they arose, in an almost identical form, by conjugation (sometimes also, for a certain time, by parthenogenesis) under certain fixed conditions as soon as they leave its body. We can thus say with Weismann, speaking philosophically, that these germinal cells continue the life of their parents, so that in reality death only destroys part of the individual, namely, that which has been specially adapted for certain exclusively individual ends. Each individual, therefore, continues to live in his descendants.
The germinal cell divides into a number of cells called embryonic, which become differentiated into layers or groups which later on form the different organs of the body. The embryonic period is the name given to the period between the exit of the germinal cell from the maternal body and the final complete development which it acquires in becoming the adult individual. During this period the organism undergoes the most singular metamorphoses. In certain cases it forms a free embryo which appears to be complete, having a special form and mode of life, but which finally becomes transformed into an entirely different sexual individual. Thus from the egg of a butterfly there first emerges a caterpillar, which lives and grows for some time, then changes to a chrysalis and finally to a butterfly. The caterpillar and the chrysalis belong to the embryonic period. During this period every animal reproduces in an abbreviated manner certain forms which resemble more or less those through which its ancestors have passed. The caterpillar, for example, resembles the worm which is the ancestor of the insects. Haeckel calls this the fundamental biogenetic law. We are not concerned here with embryology, and will content ourselves with some of the main points.
Germinal Cells. Hermaphrodites. We now come to conjugation. In order to avoid complications we will leave aside plants and speak only of animals. Among multicellular animals, sometimes in the same individual, sometimes in different individuals, occur two kinds of sexual glands, each containing one kind of cells—the male cells and the female cells. When both kinds of sexual glands occur in the same individual, the animal is said to be hermaphrodite. When they develop in two different individuals the animals are of distinct sexes. Snails, for example, are hermaphrodite. There also exist lower multicellular animals which reproduce by budding, but among which conjugation takes place from time to time. We shall not consider these animals any further, as they are too remote to interest us here.