But reproduction among unicellular organisms must be looked at from another side,—that of fertilization or, to use Weismann's term, amphimixis. It is said that isolated cells sometimes flow together when exhausted, forming a small plasmodium. This has been described in the poured-out coelomic fluid of sea-urchins where moribund cells club together and survive for a time in small groups or plasmodia, not to be confused with clots. A somewhat similar viable coalescence of amoebulae was described by Haeckel in Protomyxa, and occurs in some Myxomycetes. From such plasmodium-formation it is but a step to multiple conjugation, as seen in some Heliozoa and other Protozoa, and also in some Algae (q.v.). A giant amoeba, in the genus Pelomyxa, may be artificially built up by mingling two together with a needle, and then carefully adding a third and a fourth. Of common occurrence is the total conjugation of two indistinguishable units (isogamy) as in the large infusorian Noctiluca, or of two unequal units (anisogamy) as in V orticella. In many Protozoa, as in radiolarians, specialized reproductive units (gametes) are produced by the division of the ordinary vegetative units. They are often di morphic, and unite in pairs to form a zygote, which either grows into the original form, or proceeds to divide into many individuals.
Very suggestive, but on a special line, is the "partial conjuga tion," familiar in Paramaecium and its relatives, where the two conjugants, after a process of nuclear reduction, pointing on to polar-body formation in ova and a similar meiotic division in spermatogenesis, exchange micronuclear elements and then sep arate, the sequel being nuclear reconstruction in the ex-conju gants. This partial conjugation, obviously a sexual rather than a multiplicative process, possibly increases the vigour and cer tainly promotes the variability of the stock.
In artificially isolated "pure lines" (all descended from a single Paramaecium by generation after generation of fission) no con jugation Occurs, and one would not expect any, since they are all the same. In ideal conditions, e.g., as regards aeration, food-sup ply and the removal of waste-products, there seems to be no limit to asexual reproduction. In L. L. Woodruff's famous experi ments the sequence of vigorous asexual generations was contin ued, without conjugation, for over ten years. But in these op timum artificial conditions, and in natural conditions for certain species (e.g., Paramaecium caudatum), there is a periodic, it may be monthly, occurrence of a remarkable process (endomixis) in which the nuclear organization is disintegrated and then re-con structed. The individual slipper animalcules, to give them their popular name, behave as if they were going to conjugate with one another. As in the pre-conjugation phases, there is a scrapping of the dimorphic nuclei, but no conjugation occurs, and reorganiza tion follows. The disintegration and the re-integration here suf fice to secure the continuance of vigour. In other Protozoa, however, in which endomixis never occurs, M. Hartmann has been able to obtain a similar indefinite continuation of asexual re production. These observations of Woodruff and Erdmann have,
like most thorough observations, a significance far beyond the im mediate subject-matter. Throughout the whole gamut of organ isms there is a contest between life and death, that is to say between the individual conservation and the individual disinte grative loss of energy. To put it in another way, there are proc esses in the living organism that tend towards senescence (to be distinguished, as far as may be, from the diurnal running down of the clock) ; and there are counteractive processes that make for repair, recuperation, and rejuvenescence. In short, there are processes of aging and processes of regaining youth.
In senescence it is not the living matter itself that gets worn out ; it is rather the less labile framework of the cells ; the fur nishings of the laboratory, rather than the workers. This universal senescence-versus-rejuvenescence contest, is to be distinguished as far as may be, not as if they were au fond different, from the nor mal and continual recuperation of katabolism by anabolism. An organism may be balancing expenditure and income every day, and yet there may be a serious depreciation of property. This is senescence, and there are various ways in which rejuvenescence processes stave off the evil day of insolvency. In Paramaecium one of these rejuvenescent processes is endomixis, and it may be that in the subtler and more intimate modes of reproduction at higher levels, some rejuvenescence is effectively secured by re arrangements. In Polyzoa, for instance, there is a collapse of the fatigued individual into a "brown body," from a bud of which, after rest and re-integration, a vigorous new individual arises.
Modes of Reproduction in Multicellular Organisms.— (A) Many forms of asexual increase occur in multicellular plants and animals, with this in common that a considerable por tion of the parent is separated off, though not necessarily liberated, to form a new individual. Thus the freshwater Hydra gives off buds; a sea-anemone may split longitudinally into two; a nemer tine worm may break into several viable pieces ; there are two or three starfishes that actually multiply by separating off their arms; a liverwort may produce minute multicellular gemmae which float away in runlets of rain; a tiger-lily drops its bulbils; and a strawberry plant sends out runners, which root and form inde pendent individuals at well-spaced intervals. In many cases the asexual multiplication leads to the formation of physically con tinuous colonies, as in zoophytes and corals, Polyzoa and com pound tunicates; while many hydroids, such as Melia, illustrate the formation of polypoid buds which remain members of the colony, and medusoid buds which are set adrift as sexual swim ming-bells. It is certain that a knowledge of metabolic gradients (see AXIAL GRADIENTS) will explain, as is already being indicated, why there should be at particular places in the organism lines of weakness, or reductions in the intensity of metabolism, or pro cesses of cytolysis, which bring about the separation of a bud or a bulbil, or the breaking of a worm into viable pieces.