most highly organized of Protozoa; here the individuals which conjugate are only distinguished from ordinary individuals of the species by the fact that their nuclei have undergone very complicated processes of reduction and nuclear elimination. In these forms there is also no difference between young and adult individuals, beyond scarcely perceptible differences of size between individuals about to divide and those that are the products of recent division, so that these species are practically monomorphic in the active condition. In forms, however, which, like Vorticelia, are of sessile habit, small free-swimming individuals are liberated which seek out and conjugate with the ordinary sessile individuals. Here we have an instance of a morphological differentiation of the gametes which is clearly adaptive to the life-conditions of the species. In other Protozoa there may be, as already stated, differences, more or less pronounced, between young and adult individuals, and syngamy may take place either between young individuals (microgamy) or between adults (macrogamy); the gametes may be in either case ordinary individuals of the species, not specially differentiated in any way, or on the other hand they may be differentiated from ordinary individuals, while still similar and isogamic amongst themselves; or, finally, they may be anisogamic; that is to say, differentiated into two distinct types. Thus in the Radiolaria, for example, an adult individual breaks up by a process of sporulation into numerous minute flagellated swarm-spores; these may be all of one kind, termed isospores, which develop directly without undergoing syngamy; or they may be of two kinds, termed anisospores, both different in their character from the isospores, and incapable of development without syngamy.
When the gametes are differentiated the divergence between them almost always follows parallel paths. One gamete is distinguished by its smaller size, its greater activity, and its comparative poverty in granules of reserve food-material; hence it is termed the microgamete. The other gamete is distinguished by its greater bulk, its pronounced sluggishness and inertness, and its tendency to form and store up in the cytoplasm reserve nutriment of one kind or another; hence it is termed the macrogamete, or, as some prefer to write it, the megagamete (better megadogamete). When these differences are very pronounced, as, for instance, in the Coccidia and other Sporozoa, a condition is reached which is practically indistinguishable from that seen in the sperm and ova of the Metazoa. Hence the microgamete is generally regarded as male, the macrogamete as female; and these terms may be conveniently used, although they do not in themselves imply more than would the words positive and negative, or any other pair of terms expressive of a fundamental contrast. The microgamete may become reduced to a mere thread of chromatin, which may possess one or two flagella for purposes of locomotion, as in Coccidia, &c., or may move by serpentine movements of the whole body, which resembles in its entirety a flagellum, and is often wrongly so termed. In contrast with the microgamete, its correlative, the macrogamete, tends to become a bulky, inert body, often with great resemblance to an ovum, its cytoplasm dense and granular, packed with reserve food-materials as an egg contains yolk, and without organs of locomotion or capacity for movement of any kind. Hence the macrogamete is the passive element in syngamy, which requires to be sought out and “fertilized” by the active microgamete, a division of labour perfectly analogous to that seen in the male and female gametes of Metazoa. In those cases where syngamy takes place by interchange of nuclear substance between two gametes which remain separate from one another, as in the Infusoria, each gamete forms two pronuclei, which are distinguished by their behaviour as the active and passive pronuclei respectively. The active pronucleus of each gamete passes over into the body of the other and fuses with its passive pronucleus to form a synkaryon. A similar method of procedure occurs also in Amoeba coli, according to F. Schaudinn.
When gametes are not very highly specialized they may still retain the power of multiplication by division possessed by ordinary individuals, so long as they have not undergone the process of nuclear reduction preliminary to syngamy. If, however, the gametes are highly specialized they may forfeit the power of multiplication. In this respect the microgametes are worse off than the other sex; on account of the great reduction of the body-protoplasm, and the entire absence of any reserve materials, they must either fulfil their destiny as gametes or die off. The macrogametes, on the other hand, with their great reserves of cytoplasm and nutriment, are more hardy than any other forms of the species, and are able to maintain their existence in periods of famine and starvation when all other forms are killed off. Moreover they may regain the power of multiplication by a process of parthenogenesis, a term originally applied in the Metazoa to cases where a germ-cell of definitely female character, that is to say an ovum, acquires the power of reproduction without fertilization by syngamy. A macrogamete multiplying by parthenogenesis first goes through certain nuclear changes whereby it is set back, as it were, from the female to the indifferent condition, and it is then able to multiply by fission like any ordinary, non-sexual individual of the species. Parthenogenesis has been described by F. Schaudinn in the malarial parasites and in Trypanosoma noctuae. In both cases the female forms are able to persist under adverse conditions after all other forms have perished, and then by parthenogenesis they may multiply when conditions are more favourable, overrun the host again, and cause a relapse of the disease of which they are the cause. S. v. Prowazek has described in Herpetomonas muscae-domesticae an analogous process of multiplication on the part of male individuals, and has coined the term etheogenesis for this process, but the statement needs confirmation, and as a general rule the microgamete is quite incapable of independent reproduction under any circumstances.
It is often found that not only are the gametes differentiated, but that their immediate progenitors may also exhibit characters which mark them off from the ordinary or indifferent individuals of the species. In such cases the parent-forms of the gametes are termed gametocytes, and they may differ amongst themselves in characters which render it possible to distinguish those destined to produce microgametes from those which will produce the other sex. The parent-individuals of the microgametes, or microgametocytes, are distinguished as a general rule by clearer protoplasm, free from coarse granulations, and a larger nucleus, more rich in chromatin. The macrogametocytes, on the other hand, usually have coarsely granular cytoplasm, rich in reserve food-stuffs, and a relatively small nucleus. The gametocytes produce the gametes by methods that vary according to the degree of specialization of the gametes. In isogamous forms, of which good examples are furnished by many Gregarines (q.v.), the gametes are produced by a process of sporulation on the part of the gametocytes, a certain amount of residual protoplasm being left over. In forms with pronounced anisogamy, for instance, Coccidia or Haemosporidia, the microgametes are produced by sporulation in which almost the whole mass of the body of the gametocyte may be left over as residual protoplasm, together with some portion of the nucleus; in the other sex, however, the process of sporulation may be altogether in abeyance, and the macrogametocyte becomes simply converted into the macrogamete after going through a process of nuclear reduction.
The gametocytes may, however, possess the power of multiplication without change of character for many generations; or, to put the matter in other words, the sexual differentiation may be apparent not merely in the generation immediately preceding the gametes, but in many generations prior to this. Thus a given species may consist of three different types of adult individuals, male, female and indifferent, each multiplying in its own line. Complicated alternations of generations are the result, and if at the same time there is a well-marked difference between young and adult forms of the species the height of polymorphism is reached. Very commonly a double series of generations occurs, the non-sexual or indifferent forms multiplying apart from the sexually differentiated individuals and the generations immediately descended from them; in such cases the