Page:Popular Science Monthly Volume 79.djvu/580

576 Arcella vulgaris, a common fresh-water shelled rhizopod, has a much more simple life-history. The nucleus of the young form soon divides, so that most Arcella specimens contain two primary nuclei (Fig. 9). From the very outset, furthermore, each nucleus secretes a chromatin substance which collects in a zone about the periphery of the cell. This substance is not granular like the chromidia, but has a similar origin from the nucleus, and has the same germ-plasmic fate as chromidia, so that Hertwig was justified in calling it a "chromidial net." When the organism is mature, minute nuclei condense out of the substance of this network, hundreds of them being formed (Fig. 10). As in Polystomella, each nucleus becomes surrounded by a zone of protoplasm, and, finally, a large number of small swarmers emerge from the shell mouth, leaving behind in the shell the two primary nuclei and a portion of the protoplasm as a degenerating residue. The swarmers are dimorphic,

some are macrogametes, some microgametes (Fig. 11). These fuse two by two, a macrogamete with a microgamete, and the resulting zygote develops into the normal form.