The process we have just examined is called "mitosis," "karyomitosis," or "karyokinesis"; and the nucleus is said to undergo "indirect" division, as compared to "direct" division by mere constriction. In an intermediate mode, common to many Protista, the nuclear wall persists throughout the whole process, though a spindle is constituted within, and chromosomes are formed and split: the division of the nucleus takes place, however, by simple constriction, as seen in the Filose Rhizopod _Euglypha_ (Fig. 8). [Illustration: FIG. 8.—Fission with modified karyokinesis in the Filose Rhizopod _Euglypha_. A, outgrowth of half of the cytoplasm, passage of siliceous plates for young shell outwards; B, completion of shell of second cell, formation of _intra_-nuclear spindle; C, D, further stages. (From Wilson, after Schewiakoff.)] In many Sarcodina and some Sporozoa the nucleus gives off small fragments into the cytoplasm, or is resolved into them; {30}they have been termed "chromidia" by E. Hertwig. New nuclei may be formed by their growth and coalescence, the original nucleus sometimes disappearing more or less completely. In certain cases the division of the nucleus is not followed by that of the cytoplasm, so that a plurinucleate mass of protoplasm results: this is called an "apocyte"; and we find transitional forms between this and the uninucleate or true cell. Thus in one species of _Amoeba_ (_A. binucleata_) there are always two nuclei, which divide simultaneously to provide for the outfit of the daughter-cells on fission. Again, we find in some cases that similar multinucleate masses may be formed by the union of two or more cells by their cytoplasm only: such a union is termed "permanent plastogamy," and the plurinucleate mass a "plasmodium."[42] Here again we find intermediate forms between plasmodium and apocyte, for the nuclei of the former may divide and so increase in number, without division of the still growing mass. Both kinds of plurinucleate organisms are termed "coenocytes" without reference to their mode of origin. The rhythm of cell-life that we have just studied is called the "Spencerian" rhythm. Each cell in turn grows from half the bulk of its parent at the time it was formed to the full size of that parent, when it divides in its own turn. Rest is rare, and assumed only when the cell is exposed to such unfavourable external conditions as starvation, drought, etc.; it has no necessary relation to fission.