PROTOZOA. 151 



Protozoa is, primarily, mere fission or division of the cell-sub- 

 stance. In some instances this division is little more than an 

 irregular breaking up or fragmentation of the protoplasm. In 

 others, one or more buds may arise from the parent cell. A 

 more typical method is by the equal division of the parent into 

 two new individuals. In still other instances, especially among 

 the Sporozoa, there is the formation of a cyst, within which 

 the protoplasm rearranges itself in numerous small bits which 

 finally break from the cyst as new individuals. In all such 

 cases the old nuclear material is distributed among the daugh- 

 ter individuals. There are indications that the process of 

 division carried on for a long time without cessation results in 

 a gradual loss of the vitality of the stock. There are two ways 

 in which this untoward result is overcome, so that a kind of 

 rejuvenation occurs. In the first place, a thick wall may be 

 formed and a period of rest ensue (encystment) . Or in the 

 second place, there may be a temporary (Paramecium) or 

 permanent (Volvox, Vorticella) union of two or more in- 

 dividuals. This is conjugation. ' The essential thing in con- 

 jugation seems to be the introduction of new nuclear matter 

 into the cell. The conjugation-cells (gametes^ may be alike 

 (Paramecmm) , or diverse (Vorticella or Volvox). Parame- 

 cium may reproduce for many generations by division, and 

 then two individuals may conjugate, exchange certain nuclear 

 elements, and separate, beginning once more their process of 

 division. There is here no sign of sexual dimorphism. In the 

 colonial species however, as Vorticella and Volvox, there is 

 the union and permanent fusion of individuals (cells), dis- 

 tinctly different in form and size, to produce the new indi- 

 vidual. This is much like the dimorphism found in the sexual 

 cells in the Metazoa or many-celled animals, and illustrates 

 heterogamy (see 98). Consult Figs. 6, 67, 71. 



195. History. The existence of the Protozoa was prac- 

 tically unknown until the compound microscope came into use. 

 A naturalist of Holland first discovered the Infusoria, and 



