In the Coccidia, which are extremely simple unicellular organisms, equipped, however, with a nucleus, the adaptations relating to amphimixis are more extensive and more complex than in the Rhizopods. For while in the latter the two conjugating cells are absolutely alike in external appearance, in the former the male cell is distinct from the female, and indeed the differences are as marked as those that usually occur in multicellular animals.
Fig. 121. Life-cycle of Coccidium lithobii, a cell-parasite of the centipede Lithobius; after Schaudinn. 1, a 'sporozoite'; 2, the same penetrating into an intestinal epithelial cell; 3, the same growing into a 'schizont' capable of division; 4, the same dividing, and 5, breaking up into numerous pieces which separate from the 'residual body' in the centre, and either, as in 1, migrate into epithelial cells and repeat the history, or pass on to the phase of sexual reproduction. In the latter case, after eliminating a portion of the nucleus (reduction) in 6 and 6 a, they form the 'macrogamete' (the ovum); or within the mother-cell they produce microgametes (or sperm-cells), 7 and 7 a. The penetration of a sperm-cell into an egg-cell (amphimixis) is shown in 8, the fertilized egg-cell (9) becomes the so-called oocyst or permanent spore, from which by repeated division (10 and 11), new sporozoites, as in 1, arise, and begin the cycle afresh.
We owe our present knowledge of these processes especially to Schuberg, Schaudinn, and Siedlecki, and, because of their theoretical importance, I should like to summarize the essential points.
One of these Coccidia lives in the intestinal cells of a small centipede, Lithobius; in Fig. 121 the parasite is shown as a so-called 'Sporozoite,' that is, as a minute sickle-shaped cell, which at first moves freely about the intestine of the host (1), but then soon penetrates into an epithelial cell (2). There it grows to a spherical shape (3), and then, after having devoured the cell, it gives rise, by a peculiar process of division (Schizogony), to a number of very minute nucleated pieces, again sickle-shaped, the Schizonts, each of which bores its way into an epithelial cell as in 2, and follows the same path of development, so that a large number of cells in the intestine of the same host are attacked in this manner. But there is still another mode of reproduction, with which amphimixis is associated, which leads directly to the formation of 'lasting' germs which are enclosed in a capsule or cyst, reach the exterior with the excrement of the host, and thus spread the infection to other centipedes. The Schizonts which take this course develop into so-called macro-gametes and microgametes, the former being the female, the latter the male germ-cells. Then follows the penetration of a male gamete, actively motile because of its two flagella, into the female gamete (8). Amphimixis is accomplished, and the product of the fusion of the two sex-cells (9) surrounds itself with a thinner cyst, within which it multiplies by twofold division into four cells (10). These are the 'lasting' spores, which may dry up within the voided excrement of the centipede (11), and if they be eaten by another animal of the species, they infect it, for the sporozoites which have been formed by the previous divisions creep out, and in form 1 begin the life-history anew.
We have thus an alternation of four generations which are all unicellular, and of which one series (1-5) shows multiplication by fission, while the other (6-11) includes, besides multiplication by fission and as a condition of this, the process of amphimixis. Amphimixis must occur in order that the formation of 'lasting' spores and new sporozoites may result. We have thus a regular alternation of 'asexual' and 'sexual' reproduction, and the latter shows great resemblance to that of multicellular organisms. The macrogamete corresponds to the ovum, the microgametes to the spermatozoa, and they resemble these also in their greater numbers and in their structure.
But the resemblance goes even further. The ovum is much larger than the sperm-cell, and undergoes a kind of reduction of its nuclear substance; shortly before fertilization the ovum-nucleus ('the germinal vesicle') comes to the surface—just as in the case of animal ova—bursts, and extrudes a part of its substance in the form of a sphere (Fig. 121, 6 and 7). A reduction of the nuclear substance in the male cell has not been demonstrated in all cases, but in one of the Lithobius-Coccidia, Adelea ovata, the relatively large microgamete (the sperm-cell, Fig. 122, Mi) places itself close to one pole of the female macrogamete (the egg-cell) and then divides twice in succession, so that four small cells arise (Fig. 122, A-C); of these only one penetrates into the egg-cell (D, ♂K) and unites with it, the other three come to nought (D, Mi). What a surprising resemblance this bears to the twofold division of the mother sperm-cell in multicellular animals, through which the number of chromosomes is reduced to half! In the conjugation itself the thread-like chromosomes of the female nucleus are plainly recognizable, while those of the male remain coiled up (Fig. 122, D).
That the nuclear substance can be separated into chromosomes (ids) even in lowly unicellular organisms was probably first demonstrated by R. Hertwig for Actinosphærium, a Heliozoon or freshwater sun-animalcule, then by Lauterborn in regard to Diatoms, by Blochmann for an indigenous Rhizopod, Euglypha, and by Ishikawa for the marine Noctiluca. Fresh cases have been added in the last decade, so that we can now say that a considerable number of unicellulars, from the ciliated Infusorians and lower Algæ down to the Coccidia and Diatoms, exhibit a germ-plasm composed of ids. These structures behave in the same way as those in higher organisms, and Berger was able to demonstrate, in 1900, in the case of a Radiolarian, their multiplication by spontaneous splitting.
