6o 



THE PROTOZOA 



From an a priori point of view, there is no reason why the reproduction 

 of Protozoa by simple division should not go on indefinitely. The 

 mechanism of metabolism, growth, and reproduction is present, and 

 the cell appears therefore to be self-sufficient. Nevertheless, Biitschli, 



Maupas, and many others have 

 shown that, in many cases at least, 

 these divisions can be maintained 

 only for a certain period or number 

 of generations, after which the in- 

 dividuals become weaker, deformed, 

 and finally die out, an exhausted 

 race (Fig. 29). If two individuals 

 conjugate while in this enfeebled 

 condition, the result is a rejuve- 

 nescence or renewal of youth in 

 both cases, and each of the conju- 

 gants enters upon a new cycle of 

 cell-generations (Fig. 30). The 

 union of two cells is accomplished 

 in various ways. In some cases it 

 is by the absolute and permanent 

 fusion of two individuals; in other 

 cases there is a union for a short 

 Fig. &.- onychodromus grandis St. in ^ followed by a separation; in 



conjugation. [MAUPAS.] m, micronuclei in 



division. still other cases the entire cell does 



not conjugate, but develops a great 



number of swarm-spores, which again may be of equal or unequal 

 size, and which conjugate usually by total fusion ; finally, sexual 

 reproduction, almost as highly differentiated as in the Metazoa and 

 Metaphyta, is found in some Sporozoa and in the complex colonies of 

 Flagellidia. 1 



A very interesting controversy has arisen over the question whether 

 Protozoa ever succumb to old age. Ehrenberg was apparently the 

 first to suggest that because of their reproduction by division, a process 

 in which no portion of the original organism is lost, the Protozoa are 

 potentially immortal. This conception was greatly expanded by 

 Weismann ('84) and formed the basis of a suggestive essay in which 

 he maintained that the Protozoa are too simply organized to die a 

 natural death and that death is first observed in multicellular animals 

 and plants. Dujardin ('41), opposed to so many of Ehrenberg's 

 theories, was equally opposed to this, and although he was unable to 

 disprove the theory, he suggested that simple division cannot continue 



1 Cf. Chapter VII, p. 229. 



