122 THE PROTOZOA 



ill size, and then grows slowly until the next division-period is 

 reached. As a result of the slow " functional growth " of the 

 nucleus, a disproportion between the mass of the nuclear substance 

 and that of the cytoplasm is brought about, producing a condition 

 of tension between the nucleus and the cytoplasm (" Kernplasma- 

 Spannung "). When the tension reaches a maximum, the nucleus 

 acquires the power of growing rapidly at the expense of the cyto- 

 plasm, and this " division-growth " leads to the fission of the cell, 

 restoring the standard balance of nucleus and cytoplasm. Relative 

 increase of the nuclear substance retards the cell-division, and 

 brings about increase in the size of the cell ; relative decrease of the 

 nuclear mass has the opposite effect. 



2. Division of the, Cell. A distinction has been drawn above 

 between binary fission, or division of the body into two, and mul- 

 tiple fission into many parts simultaneously. The daughter-indi- 

 viduals produced in either case may be similar to the parent-indi- 

 vidual in all respects except size, or may differ from it in lacking 

 more or fewer of its characteristic parts and organs, which are then 

 formed after the daughter-individuals are set free. In extreme 

 cases one or more of the daughter-individuals may possess, when 

 first liberated, no structure more elaborated than the essential 

 parts of a cell, cytoplasm and nucleus or chromidia ; in such cases 

 the daughter is termed a " bud," and the process of fission by which 

 it arises is termed " budding " or gemmation, distinguished further as 

 " simple gemmation " when only one bud is formed at a time, and 

 " multiple gemmation " when many arise simultaneously. In many 

 cases of multiple gemmation the parent- organism does not survive 

 the process, but breaks up almost completely into buds, leaving 

 only a greater or less amount of residual protoplasm, which degene- 

 ates and dies off ; budding of this kind is termed sporulation. 



In binary fission, when the organism is of simple structure, as in 

 the case of amcebse, the division is equally simple. After division 

 of the nucleus, the two daughter-nuclei travel apart, and the body 

 follows suit, by flowing, as it were, in two opposite directions, 

 forming two smaller individuals each with a nucleus, and con- 

 nected at first by a protoplasmic bridge, which soon snaps and is 

 drawn in. The contractile vacuole, if present, is taken over by 

 one of the two daughter-individuals, while the other forms a new 

 vacuole ; in many cases the normal number of contractile vacuoles 

 is doubled before division begins. 



In forms of more complicated structure, the division also becomes 

 a more complex process. Where the body-form is definite, the 

 plane of cleavage bears usually a constant relation to it. Thus, in 

 Ciliata the division of the body takes place typically transversely 

 to its longitudinal axis, except in the order Peritricha. In Flagel- 



