158 ZOOLOGY 



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. 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. These 

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

 cases the old nuclear material is distributed among the daughter 

 individuals. 



There are some 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 three ways in 

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

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

 formed and a period of rest ensue (encystment). It appears 

 also that new vitality may be given to a culture of paramecia 

 by rhythmic changes in the conditions of life, particularly in 

 the foods. Or in the third place, there may be a temporary 

 (Paramecium) or permanent (Volvox, Vorticella) union of two or 

 more individuals. This is conjugation. The essential thing in 

 conjugation seems to be the introduction of new nuclear matter 

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

 (Paramecium), or diverse (Vorticella or Volvox). 



Paramecium may reproduce for many generations by divi- 

 sion, 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 Paramecia themselves. It has been discovered, however, 

 that the portion of the nucleus which passes out of each con- 

 jugant into the other is smaller than that with which it unites. 

 It might therefore be considered a male nucleus. 



In the colonial species, as Vorticella and Volvox, there is 

 the union and permanent fusion of the whole protoplasm of 

 individuals (cells), distinctly different in form and size, to 

 produce the new individual. This is much like the dimorphism 



