PROTOZOA 159 



199. Reproduction. — In the Protozoa we discover methods 

 of reproduction which are to be looked upon as suggestions of 

 methods found in the Metazoa. Reproduction among the 

 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 

 (budding) . A more typical method is by the equal division of the 

 parent into two new individuals. In still other instances, espe- 

 cially among the Sporozoa, there is the formation of a cyst, 

 within which the protoplasm rearranges itself in numerous small 

 bits (spore formation). These finally break from the cyst as 

 new individuals. In all such cases the old nuclear material is 

 distributed among the daughter individuals. Fission, budding 

 and spore formation are asexual methods of reproduction. 



At irregular intervals there may be a temporary (Parame- 

 cium) or permanent (Vorticella) union of two or more individ- 

 uals. This is conjugation. The essential thing in. conjugation 

 seems to be the introduction of new nuclear matter into the 

 cell. The full value of this nuclear fusion is still in controversy. 

 The conjugation-cells (gametes) may be alike (Paramecium), 

 or diverse (Vorticella). 



Paramecium may reproduce for many generations by divi- 

 sion, and then two individuals may conjugate (Fig. 69), ex- 

 change certain nuclear elements, and separate, — beginning once 

 more their process of division. There is here no sign of sexual 

 dimorphism in the Paramecia themselves. It has been discov- 

 ered, however, that the portion of the nucleus which passes out 

 of each conjugarjt into the other is smaller than that with which 

 it unites. ' 



In many Protozoa, as Vorticella and Volvox, there is the 

 union and permanent fusion of the whole protoplasm of cells, 

 distinctly 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 §101). Consult Figs. 7, 69, 73. 



200. History. — The existence of the Protozoa was prac- 

 tically unknown until the compound microscope came into use. 



