THE INFUSORIA 187 



may be as many as four such divisions, with the production of 

 sixteen individuals in twenty-four hours. The rate of this division 

 is determined by external conditions, such as food and tempera- 

 ture, and by certain internal factors. This production of " orphan 

 twin sisters " continues until interrupted by conjugation or endo- 

 mixis. 



Although P. aurelia and probably other paramcecia may con- 

 tinue to live indefinitely without the sexual reproduction that is 

 effected by conjugation, this process apparently occurs at more or 

 less frequent intervals under natural conditions as well as in the 

 laboratory. In some cultures that were long continued (Calkins, 

 P. caudatum), it was observed that the need for conjugation 

 occurred at intervals of some two hundred generations. In others 

 (Woodruff, P. aurelia), it was found that conjugation need not 

 occur even in many thousands of generations, if at all. The 

 details of conjugation in P. caudatum, after the two cells have 

 come in contact in the region of their buccal grooves, are shown in 

 Fig. 100, the explanation of which should be consulted in this 

 connection. The process differs markedly from the permanent 

 fusion of cells that occurs in Sarcodina and Mastigophora. In 

 such protozoa, conjugation consists in the complete and perma- 

 nent union of two cell bodies and of their nuclei, and is thus com- 

 parable with the union of egg and sperm in the fertilization of 

 higher animals (c/. Fig. 116, p. 233). The type of conjugation 

 that occurs in paramoecium is found only among the Infusoria, 



However, the net results are the same in both cases. Conjuga- 

 tion in one of the Sarcodina, such as amoeba, or in one of the Mas- 

 tigophora, such as euglena, results in a single cell of biparental 

 origin, which is a zygote, comparable with the zygote formed by 

 union of egg and sperm in many-celled animals. From two cells 

 one is formed. In paramoecium a similar condition is brought 

 about, since there arise, as a result of conjugation, zygotes, or cells 

 of bi-parental origin (Fig. 100). The conjugation of paramoecium 

 and other Infusoria may be said to be more efficient, however, 

 because two zygotes, instead of one, result from each conjugation. 

 Although the term " sexual reproduction " is commonly applied 

 to the conjugation of paramoecium, it wiU be noted that the 

 conjugating cells are isogametes and hence there is no evidence of 

 sexual differentiation. Attempts are sometimes made to com- 

 pare the migratory nucleus of conjugation with a sperm nucleus. 



