Page:EB1911 - Volume 22.djvu/500

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485
PROTOZOA

other hand, those resulting from recent fission will be smaller than the average; and such differences are, it need hardly be said, more pronounced when the fission is of the unequal binary type, or in cases of gemmation or multiple fission. In cases also where a given strain of a species is becoming senile, it is sometimes observed that the individuals are markedly undersized on the average.

On the other hand, it is often the case that the young individuals resulting from a recent act of multiplication may differ from adult individuals of the species, not merely in size, but in structural characters, to such an extent that their relationship to the adult forms could not be determined by simple inspection without other evidence. This is especially true of those species in which multiplication by sporulation occurs, giving rise to numerous small spores which may at first be in a resting condition, enveloped in protective sporocysts, but which sooner or later become free, motile individuals known technically as swarm-spores. Thus in many Sarcodina the adult is a large amoeboid organism which produces by sporulation a great number of relatively minute swarm-spores. These may be either, as in the common Amoeba proteus, amoeboid organisms, so-called amoebulae or pseudopodiospores, or, as in the Foraminifera and Radiolaria, flagellated organisms, so-called flagellulae or flagellispores. Sometimes, as in many Mycetozoa, amoeboid and flagellated phases may succeed each ether rapidly in the development of the swarm-spores. The familiar Noctiluca miliaris is another instance of a species which produces by sporulation numerous tiny swarm-spores quite different from the parent form in their characters. Such instances could be multiplied indefinitely amongst the Protozoa.

When the young individuals differ greatly from the adults in structure and appearance they may be regarded as larval forms, and it is interesting to note that such forms appear to be just as much recapitulative, in the phylogenetic sense, as are the larvae of many Metazoa. A striking instance is that of the Acinetaria, in which the swarm-spores produced by gemmation are ciliated, and thus betray affinities with the Ciliata which could hardly be suspected from a study of the adult forms alone. Similarly, in the genus Trypanosoma, the young forms often show a Herpetomonas-like structure which is probably of phyletic significance. The swarm-spores of Sarcodina and of Noctiluca mentioned above can, perhaps, be regarded in the same light. On the other hand, many larval forms cannot be considered as exhibiting recapitulative characters, but merely as adaptations to environment or other special life-conditions. This is especially true, as in Metazoa, of parasitic forms, subject as they are to great vicissitudes, to cope with which the most finely adjusted adaptations are necessary on the part of the organism.

3. Polymorphism in Relation to Sex.—In all Protozoa of which the life-cycle has been made known in its entire course, a process of syngamy or sexual union has been found to occur. There are still many forms in which syngamy remains to be discovered: this is true even of some groups of considerable extent. It is quite possible, therefore, that Protozoa exist in which syngamy does not occur. In view, however, of the widespread occurrence of sexual processes amongst unicellular organisms, both of animal and vegetable nature, and the fact that extended observation continually brings to light new instances of this kind, it is safer, in cases amongst the Protozoa in which syngamy is not known to occur, to explain its apparent absence by the imperfections of the present state of our knowledge, than to suppose that in such forms sexual phenomena are entirely lacking in the life-cycle.[1]

The process of syngamy, though greatly diversified in different forms, consists essentially of one and the same process in all cases; namely, the fusion of nuclear matter from two distinct individuals. Plus ça change, plus c’est la même chose! Hence true syngamy may be distinguished as karyogamy from the process of plastogamy, or fusion of the protoplasmic bodies, of frequent occurrence in many forms of Protozoa. The individuals whose nuclei undergo fusion are termed gametes. They may be in no way different from each other or from ordinary individuals of the species, or, on the other hand, they may be highly differentiated in size, form and structure. The two gametes may undergo complete fusion into one body, thus giving rise to an individual termed generally a zygote or copula, but which may bear special names in special cases (e.g. vermicule or oökinete of the malarial parasites, &c.); such a process is termed sometimes copulation. On the other hand, the bodies of the two gametes may remain distinct, and portions of the nucleus of each be exchanged between them; to this condition the term conjugation is sometimes specially applied. The act of syngamy may be performed in the free condition, or in the resting state, within a cyst.

The significance of syngamy has been much discussed, and it is very difficult to make positive statements upon this point. By comparing the life-cycles of different forms it is found that syngamy sometimes precedes, sometimes follows, a period of great reproductive activity on the part of the organism. Thus in such a form as Noctiluca, syngamy between two full-grown individuals is followed by rapid sporulation and the production of a swarm of young individuals; on the other hand, in Foraminifera and Radiolaria, rapid sporulation of adult individuals produces a numerous progeny of young forms which may go through the process of syngamy and produce zygotes that simply grow into the adult form. Comparing these two types of development, instances of which might be greatly multiplied, it is seen that in one case syngamy follows a period of growth and precedes a period of proliferation in the life-cycle, and that in the other case exactly the reverse is true. Hence it follows that syngamy must not be regarded as in any way specially connected with reproduction, but must be considered in its relation to the life-cycle as a whole, and in those instances in which syngamy is followed by increased reproductive activity the explanation must be sought in the general physiological effects of the sexual process upon the vital powers of the organism.

In the Metazoa the sexual process is always related to the production of a new individual, that is to say, of a multicellular organism for which there is no analogy amongst the Protozoa, although an approach to the Metazoan condition is seen in colony-forming Flagellata, such as Volvox and its allies. The reproduction of Protozoa is analogous to the ordinary process of cell-division and multiplication which is going on at all times in the bodies of the Metazoa, and which can be observed in the production of the gametes; that is to say, in the period of the life-cycle immediately preceding the sexual process in the Metazoa, just as much as in the developmental phases which follow syngamy and result in the building up of a new Metazoan individual. Hence, so far as the Protozoa are concerned, the phrase “sexual reproduction” is an incongruous combination of words; reproduction and sex are two distinct things, not necessarily related or in any direct causal connexion; and in order to arrive at any theory of sex it is necessary first of all to clear away all misconceptions or preconceived notions arising from analogies with the multicellular Metazoan individual.

Many observations indicate that the vital powers of the Protozoa become gradually weakened, and the individual tends to become senile and effete, unless the process of syngamy intervenes. The immediate result of the sexual union is a renewal of the vitality, a rejuvenescence, which manifests itself in enhanced powers of metabolism, growth and reproduction. These facts have been most studied in the Ciliata. It is observed that if these organisms be prevented from conjugating with others of their kind they become senile and finally die off. It has been found by G. N. Calkins, however, that if the senile individuals be given a change of medium and nourishment, their vigour may be renewed and their life prolonged for a time, though not indefinitely; there comes a period when artificial methods fail and only the natural process of syngamy can enable them to prolong their existence. The results obtained by Calkins are of great interest, as indicating that under special conditions

  1. It will be shown below, however, that in some species syngamy may perhaps be secondarily in abeyance.