I 



PROTOZOA chap. 



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 " Plas- 

 modium." 1 Here again we find intermediate forms between Plas- 

 modium 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. Eest 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. 



Multiple fission or brood-formation. We may now turn to 

 a new rhythm, in strong contrast to the former: a cell after 

 having attained a size, often notably greater than its parents, 

 divides : without any interval for growth, the daughter - cells 

 again divide, and this may be repeated as many as ten times, 

 or even more, so as to give rise to a number of small cells 4, 

 8, 16 1024, 2 etc., respectively. Such an assemblage of small 

 cells so formed is called a brood, and well deserves this name, 

 for they never separate until the whole series of divisions is 

 completed. By this process the number of individuals is rapidly 



1 Temporary plastogamy is a process found in some Foraminifera, where two 

 organisms unite by their cytoplasms so that there can be complete blending of 

 these, while the nuclei remain distinct : they ultimately separate again. In the 

 conjugation of the Infusoria, the union of the cytoplasms is a temporary 

 plastogamy (see p. 148 f. ). 



2 See Figs. 9, 29, 31, 34, etc., pp. 54, 89, 95, 101. 



