Page:Popular Science Monthly Volume 79.djvu/575

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PSM V79 D575 Coalesced cells merging with the stationary protozoa cell.png
Fig. 3.After Calkins.

cells which now undergo, separately, the later stages of development. These stages consist in the division of the fertilization nucleus and new formation from the division products, of the new macronucleus and the new micronucleus (Fig. 4).

After such a process of fertilization it would seem that the individuals are pretty much as they were before except for the complete reorganization of the nuclear apparatus, and there is a certain justification for the Weismann conception. But the phenomena in Paramecium and allied forms, like their cell differentiations, are highly specialized and are unlike the fertilization processes in the majority of Protozoa.

The enormous group of Sarcodina, including more than four thousand species of Radiolaria and some thousand or more species of Foraminifera, Heliozoa and Rhizopods, presents a fairly uniform picture of the germ plasm and the processes of fertilization. For purposes of illustration and comparison I will describe two types selected from this great group of forms—one a marine foraminiferon, Polystomella, crispa, the other a common fresh-water rhizopod Arcella vulgaris.

So far as known, each species of Foraminifera exists in two forms known as the microsphæric and the megalosphæric forms, so called because of the small and large size of the central or initial chamber of the shell (Figs. 5 and 6). These two forms correspond with the asexual and the sexual generations of metagenetic hydrozoa, the microsphæric