Page:EB1911 - Volume 19.djvu/121

 LIFE-HISTORY OF THE MYCETOZOA

Endosporeae. We may begin our survey of the life-history at the point where the spores, borne on currents of air, have settled among wet decaying vegetable matter. Shrunken when dry, they rapidly absorb water and resume the spherical shape which is found in nearly all species. Each is surrounded by a spore wall, sheltered by which the protoplasm, though losing moisture by drying, may remain alive for as many as four years. In several cases it has been found to give the chemical reaction of cellulose. It is smooth or variously sculptured according to the species. Within the protoplasm may be seen the nucleus, and one or more contractile vacuoles make their appearance. After the spore has lain in water for a period varying from a few hours to a day or two the wall bursts and the contained protoplasm slips out and lies free in the water as a minute colourless mass, presenting amoeboid movements (fig. 1, 𝑐). It soon assumes an elongated piriform shape, and a flagellum is developed at the narrow end, attaining a length equal to the rest of the body. The minute zoospore, thus equipped, swims away with a characteristic dancing motion. The protoplasm is granular within but hyaline externally (fig. 1, 𝑑). The nucleus, lying at the end of the body where it tapers into the flagellum, is limited by a definite wall and contains a nuclear network and a nucleolus. It often presents the appearance of being drawn out into a point towards the flagellum, and a bell-like structure [first described by Plenge (27)], staining more darkly than the rest of the protoplasm, extends from the base of the flagellum and invests the nucleus (fig. 2, 𝑎 and 𝑐). The other end of the zoospore may be evenly rounded (fig. 1, 𝑑) or it may be produced into short pseudopodia (fig. 1, 𝑒). By means of these the zoospore captures bacteria which are drawn into the body and enclosed in digestive vacuoles. A contractile vacuole is also present near the hind end. Considerable movement may be observed among the granules of the interior, and in the large zoospores of Amaurochaete atra this may amount to an actual streaming, though without the rhythm characteristic of the plasmodial stage.

Other shapes may be temporarily assumed by the zoospore. Attaching itself to an object it may become amoeboid, either with (fig. 1, 𝑓) or without (fig. 2, 𝑐) the temporary retraction of the flagellum; or it may take an elongated slug-like shape and creep with the flagellum extended in front, with tactile and apparently exploratory movements.

That the zoospores of many species of the Endosporeae feed on bacteria has been shown by A. Lister (18). New light has recently been thrown on the matter by Pinoy (26), who has worked chiefly with Sorophora, in which, as shown below, the active phase of the life-history is passed

mainly in the state of isolated amoebae. Pinoy finds that the amoebae of this group live on particular species of bacteria, and that, the presence of the latter is a necessary condition for the development of the Sorophora, and even (as has been recognized by other workers) for the hatching of their spores. Pinoy's results indicate, though not so conclusively, that bacteria are likewise the essential food of the Euplasmodida in the early phases of their life-history. The zoospores do, however, ingest other solid bodies, e.g. carmine granules (Saville Kent, 15).

The zoospores multiply by binary fission the flagellum being withdrawn and the nucleus undergoing mitotic division, with the formation of a well-marked achromatic spindle (fig. 3).

It is probable that fission occurs more than once in the zoospore stage; but there is not satisfactory evidence to show how often it may be repeated.

At this, as at other phases of the life-history, a resting stage may be assumed as the result of drying, but also from other and unknown causes. The flagellum is withdrawn and the protoplasm, becoming spherical, secretes a cyst wall. The organism thus passes into the condition of a microcyst, from which when dry it may be awakened to renewed activity by wetting.

At the end of the zoospore stage the organism finally withdraws its flagellum and assumes the amoeboid shape. It is now known as an amoebula. The amoebulae become endowed, as was first recognized by Cienkowski, with mutual attraction, and on meeting fuse with one another. Fig. 4 represents a group of such amoebulae. Several have already united to form a common mass, to which others, still free, are converging. The protoplasmic mass thus arising is the plasmodium. The fusion between the protoplasmic bodies of the amoebulae which unite to form it is complete. Their nuclei may be traced for some time in the young plasmodium and no fusion between them has been observed at this stage (20). As the plasmodium increases in size by the addition of amoebulae the task of following the fate of the individual nuclei by direct observation becomes impossible.

The appearance of an active plasmodium of Badhamia utricularis, which, as we have seen, lives and feeds on certain fungi, is shown in fig. 5. It consists of a film of protoplasm, of a bright yellow colour, varying in size up to a foot or more in diameter. It is traversed by a network of branching and anastomosing channels, which divide up and are gradually lost as they approach the margin where the protoplasm forms a uniform and lobate border. Elsewhere the main trunks of the network may lie free with little or no connecting film between them and their neighbours. The plasmodia of other species, which live in the interstices of decaying vegetable matter, are less easily observed, but on emerging on the surface prior to