Page:Popular Science Monthly Volume 79.djvu/581

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In Arcella, therefore, as in Polystomella, there is a definite germinal protoplasm composed essentially of chromatin which gives rise to new cycles of organisms, while a portion of the original organism, including the primary nucleus and a quantity of protoplasm, degenerates and dies. We can speak of germinal and somatic protoplasm in these cases, and PSM V79 D581 Dimorphic cell swarmers.pngFig. 11.After Elpetiewsky with equal right in all of the thousands of allied species of protozoa, as well as in the case of any higher animal.

A still more interesting case of specific germ-plasm formation is given by a type of Gregarine belonging to a group of parasitic Sporozoa. In all of the Gregarines there is a segregation of the germ plasm and a residual somatic protoplasm which degenerates and dies a somatic death. One of the most striking illustrations of this type is the case of Ophryocystis mesnili, a parasite of beetles. Unlike the cases cited above, most of the gregarines do not reproduce asexually. Ophryocystis, however, is one of the exceptions to the rule, the individuals reproducing by simple division until the protoplasm becomes mature, when, as in Paramecium, two cells come together in conjugation. The single nucleus of each cell divides and one of the products becomes a "somatic" nucleus, to use Leger's term, while the other daughter nucleus in each cell divides again (Fig. 12, A-G). One of these corresponds to a polar body in the metazoon egg, the other is the gamete, or germ, nucleus. In each cell this nucleus collects about itself, possibly through the secretion of nuclear material which transforms the surrounding stuff, a denser zone of protoplasm, which, with the nucleus, forms a gamete within the body of the parent cell. The two gametes thus formed fuse while in the space which their formation has left in the parent somatic, or nurse cells. The latter ultimately wither up and die. After union of the two gametes, the sporoblast gives rise to eight germs or sporozoites, each capable of developing into an ordinary vegetative form when under the proper conditions of environment (Fig. 12, H-N).

Here in Gregarines, therefore, as in rhizopods, we see a clearly defined difference between germ plasm and somatic plasm, the latter dying, as in the Metazoa, the former capable of endless development. Unlike the rhizopods, however, the germinal chromatin is retained in the primary nucleus until full maturity of the cell and does not appear in the cytoplasm in the form of chromidia.

Turning after this excursion into other fields of Protozoa, to the