Page:Popular Science Monthly Volume 79.djvu/583

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579
PROTOZOAN GERM PLASM

In Paramecium there are no gametes of this kind, but portions of the adult individuals fuse as in Ophryocystis mesnili. Unlike this Gregarine, the fusion of the adult cells is only temporary and the two parties to the conjugation do not die. During the temporary fusion there is a mutual interchange of micronuclei and a mutual fertilization, while the only portion to disintegrate and die is the macronucleus of each conjugant, and this is replaced by a specially differentiated fragment of the new micronucleus.

In ciliated protozoa such as Paramecium and its near relations the germ plasm is concentrated in the micronucleus, while the somatic plasm is represented by the macronucleus. As we have seen, the micronucleus enlarges and divides during conjugation, first into two, then these into four, all but one of the four degenerating. The fourth divides for the third time, but this time unequally (Fig. 3) into a smaller migratory and a larger stationary form. The smaller micronucleus migrates into the other cell of the pair and there unites with the stationary larger nucleus. The macronucleus then degenerates and is absorbed in each of the conjugating cells.

The aberrant conditions in these Infusoria can be interpreted if we regard the three divisions of the micronucleus as the reminiscence of a

PSM V79 D583 Large number of gametes formed in monocystis.png
Fig. 13.Original.

process of gamete formation which obtained in remote ancestral forms. A parallel, but less extreme, case is seen in the Gregarine Ophryocystis, where one gamete only is formed by the conjugating cells. This is an isolated case among these Sporozoa, for in the typical forms a great number of gametes are developed, as shown in the accompanying photograph of Monocystis (Fig. 13). The one pair of gametes in Ophryocystis, plus the nuclear derivatives of the pro-conjugants, represent the