Page:EB1911 - Volume 23.djvu/263

Rh progression the body is roughly oval with the apex truncated posteriorly and the wide anterior end forming a single anterior pseudopod. Progression chiefly takes place by a rolling over of the anterior end (fig. 3—see also ); but it may take place by the extension of a pseudopod, its attachment at the tip,

followed by its contraction to pull up the rest of the animal; this is well shown in the thecate species. Another mode is that of A. radiosa (fig. 1, 1-3), which can roll over on the tips of its stiff pseudopods. The pseudopods of the Filosa (figs. 6, 7) are branched, but less rich in granules, and less viscid than those of Foraminifera; they rarely anastomose, and never coalesce to form perforated plates.

A process whose relations to reproduction are not fully made out is that of “plastogamy,” where two or more individuals unite completely by their cytoplasm, the nuclei remaining distinct; it may be temporary or permanent: in the latter case determining, of course, a much more rapid increase of size than that due to growth. Thanks to the labours of F. Schaudinn, we now know the full life cycles of at least half a dozen species; previously we only knew with certainty of two modes of fission—equal constriction (Amoeba—fig. 1, 1-3) and bud-fission (Diffugia). As in other Sarcodina, chromidia, or fragments of nuclear substance budded off from the nucleus into the endoplasm, play an important part in many reproductive processes. Equal binary fission is common. In the thecate forms, e.g. Diffugia, Euglypha (fig. 4), this is replaced by bud-fission; half the cytoplasm passes out through the pylome, and becomes invested with its covering there; the enclosed “reserve” skeletal elements pass to the surface in order, so that the pylome of the new shell faces that of the old; the original nucleus divides in situ and one daughter nucleus passes into what we may call the bud-cytoplasm; the two daughters of the original cell, which we may call the “bud-sister” and the “stock-sister” respectively, now separate. In the plurinucleate forms a true bud-formation takes place, nucleate masses of cytoplasm being constricted off at the surface. A simultaneous resolution into uninucleate cells may affect the multinucleate species (or the multinucleate state of habitually uninucleate species), this is termed schizogony.

In Trichosphaerium (fig. 5) it occurs at the close of two