A third form of asexual reproduction is the formation of spores or "germ-cells," which are usually produced in great numbers inside the organism, then detached from it, and developed into new organisms without needing fertilization. The spores are sometimes motionless (rest-spores or paulospores); sometimes they have one or more lashes which enable them to swim about (rambling-spores or planospores). This monogenetic propagation is very common among the protists, both protophyta and protozoa. Among the latter the sporozoa (gregarinæ, coccidia, etc.) are remarkable for the passing away of the whole unicellular organism in the formation of spores; in this case and in many of the rhizopods (mycetozoa) the process coincides with manifold cell-division. In other cases (radiolaria, thalamophora) only a portion of the parental cells is used for the production of spores. Spore-formation is very common among the cryptogams; here it usually alternates with sexual propagation. The spores are generally formed in special spore-capsules (sporangia). In the flowering plants (anthophyta) sporogony has disappeared. It is found at times in the tissue-animals (in the fresh-water sponges); in this case the sporangia are called gemmulæ.
The essential feature of sexual generation is the coalescence of two different cells, a female ovum (egg-cell) and a male sperm-cell. The simple new cell which arises from the blending of these is the stem-cell (cytula), the stem-mother of all the cells that make up the tissues of the histon. But even among the unicellular protists we find in many places the beginnings of sexual differentiation; it is foreshadowed in the blending or copulation of two homogeneous cells, the gameta. We may conceive this process, or zygosis, as a peculiar and very favorable kind of growth, that is connected with a rejuvenescence of the plasm; the latter is enabled to propagate by repeated cleavage through the mixing of the two different plasma-bodies on either side (amphimixis). When these two gameta become unequal and differ in size and shape, the larger female body is called the macrogameton or macrogonidion, and the smaller, male part, the microgameton or microgonidion. Among the histona the first is called the egg-cell (ovulum), and the latter the sperm-cell (spermium, or spermatozoon). As a rule the latter is a very mobile ciliated cell, the former an inert or amœboid cell. The vibratory movements of the sperm-cells serve for approaching the ovulum in order to fertilize it.
The qualitative difference between the two copulating sexual cells (gonocyta), or the chemical difference between the ovoplasm of the female and the sperm-plasm of the male cell, is the first (and often the only) condition of amphigony; subsequently we find in addition (in the higher histona) a very elaborate apparatus of secondary structures. With this chemical difference is associated a peculiar double form of sensitive perception and an attraction based thereon, which is called sexual chemotaxis or erotic chemotropism. This "sex-sense" of the two gonocyta, or elective affinity of the male androplasm and the female gynoplasm, is the cause of mutual attraction and union. It is very probable that this sexual sense-function, akin to smell or taste, and the movements it stimulates, are located in the cytoplasm of the two sex-cells, while heredity is the function of the caryoplasm of the nucleus. (Cf. the Anthropogeny, chapters vi. and vii.)
The sexual difference between the two forms of gonoplasm, the ovoplasm of the female and spermoplasm of the male cell, is noticeable at the very beginning of sexual differentiation in the different sizes of the copulating gameta, and later in their increasing divergence as to shape, composition, movement, etc. It leads further to the distribution of the germinal regions (in which the sex-cells are formed) into two different individuals. When the ovum and the sperm-cell are produced in one and the same individual, we call this an hermaphrodite; and when they are formed in two different individuals (male and female), we call them monosexual, or gonochorists. In accordance with the various stages of individuality which we distinguished above (chapter vii.), we may indicate the following stages of hermaphrodism and gonochorism.
Some groups of protists, especially the highly organized ciliated infusoria (ciliata), are distinguished by having a separation of male and female plasm within the unicellular organism. The ciliata propagate, as a rule, in large numbers by repeated division (by indirect cell-cleavage). But this monogony has its limits, and has to be interrupted from time to time by amphigony, a rejuvenation of the plasm, which is effected by the conjugation of two different cells and the partial destruction of their nuclear matter. By conjugation is meant the partial and momentary union of two different unicellulars, while copulation is a total and permanent coalescence. When two ciliated infusoria conjugate they place themselves side by side, and connect for a time by means of a bridge of plasm. A part of the nucleus of each has already divided into two portions, one of which functions as the female standing-nucleus (paulocaryon) and the other as the male travelling-nucleus (planocaryon). The two mobile nuclei enter the plasm-bridge, and move through it, pushing against each other, into the body of the opposite cell; they then coalesce with the deeper lying standing-nucleus. When a fresh nucleus has been thus formed (by amphimixis) in each of the copulating cells, they again separate. The two rejuvenated cells have once more acquired the power to propagate for a long time by division.
This peculiar hermaphroditic formation of the cells, which distinguishes the ciliated infusoria and some other protists, and which we now know in its smallest details through the investigations of Richard Hertwig, Maupas, and others, is especially interesting because it proves that the chemical difference between the female gynoplasm and the male androplasm can be found within a single cell. This erotic division of labor is so important that formerly it was universally ascribed to two different cells. Recent accurate research, penetrating into the smallest visible processes of fertilization, has shown that the essential feature in the formation of a fresh individual (the stem-cell) is the blending of equal portions (hereditary parts) of the male and female nuclei; the caryoplasm of the two copulating cells is the vehicle of heredity from the parents. The cytoplasm of the cell-body, on the other hand, serves the purposes of adaptation and nutrition. As a rule the cell-body of the ovulum is very large, and is, as a food-store, very richly provided with albumin, fat, and other nutritive matter (food-yolk); while the cytoplasm of the sperm-cell is very small, and generally forms a vibrating lash, with which it moves along and seeks the ovum.
In most of the plants the female and male cells are produced by the same sprout, and in many of the lower animals by one and the same person. This kind of hermaphrodism in "individuals of the second order" is called monoclinism ("one-beddedness"). In many of the higher plants (monœcic stocks) and most of the higher animals we have diclinism ("two-beddedness")—in other words, the one sprout or person has only male, and the other sprout or person only female, organs—this is gonochorism of individuals of the second order. Monoclinism is generally associated with sedentary life (and often necessary for it), and diclinism with free movement. Adaptation to parasitic habits also favors monoclinism; thus, the crabs, for instance, are for the most part gonochoristic individuals, but the creeping crabs (cirripedia), which have adopted sedentary (and to an extent parasitic) habits, have become hermaphrodites in consequence. Many intestinal parasites among the lower animals (such as tape-worms, suctorial worms, wonder-snails), which live isolated lives inside other animals, have to be hermaphroditic and able to fertilise themselves if the species is to be maintained. On the other hand, many hermaphroditic flowers, although they have both sorts of sex-organs, are incapable of fertilizing themselves and have to receive this from insect visitors which carry the pollen from one flower to another.
Individuals of the third order, which we call stocks (cormi) in both the plant and animal worlds, also exhibit varying features in the sex-persons which compose them. When male and female diclinic sprouts or persons are found side by side on the same stock, we call this hermaphrodism of the cormi monœcia ("one-housedness"); this is the case with most of the siphonophora and some of the corals. Diœcia ("two-housedness") is less common: in this one stock has only male and the other only female sprouts or persons, as in poplars and osiers, most of the corals, and some of the siphonophora. The physiological advantages of crossing—the union of sex-cells of different individuals—favor progressive sex-division in the higher organisms.
A comparative study of the features of hermaphrodism and sex-division in the plant and animal worlds teaches us that both forms of sex-activity are often found in closely related organisms of one and the same group, sometimes even in different individuals of the same species. Thus, for instance, the oyster is usually gonochoristic, but sometimes hermaphroditic; and so with many other mollusks, vermalia, and articulata. Hence, the question often raised, which of the two forms of sex-division is original, is hardly susceptible of a general answer, or without relation to the stage of individuality and the place in classification of the group under discussion. It is certain that in many cases hermaphrodism represents the original feature; for instance, in most of the lower plants and many of the stationary animals (sponges, polyps, platodes, tunicates, etc.). Where we find exceptions in these groups, they are of secondary origin. It is equally certain, on the other hand, that in other cases the separation of the sexes is the primitive arrangement; as in siphonophoræ, ctenophoræ, bryozoa, cirripedia, and mollusks. In these cases the hermaphrodism is clearly secondary in the sense that the hermaphrodites descend originally from gonochorists.
It is only in a few sections of the lowest histona that the two kinds of sex-cells arise without a definite location in different parts of the simple tissue, as in a few groups of the lower algæ and in the sponges. As a rule they are formed only at definite positions and in a special layer of the tissue-body, and mostly in groups, in the shape of sexual glands (gonades). These bear special names in different groups of the histona. The female glands are called archegonia in the cryptogams, nucellus (formed from the macrosporangia of the pteridophyta) in the phanerogams, and ovaries in the metazoa. The male glands are called antheridia in the cryptogams, pollen-sacs (formed from the microsporangia of the ferns) in the phanerogams, and testicles (as spermaria) in the metazoa. In many cases, especially in aquatic lower animals, the ovula (as products of the ovaries) are discharged directly outward. But, in most of the higher organisms, special sexual ducts (gonoductus) have been formed to conduct both kinds of the gonocyta out of the organism.