Chapter 5 

 Reproduction 



THE mode of reproduction in Protozoa is highly variable among 

 different groups, although it is primarily a cell division. The 

 reproduction is initiated by the nuclear division in nearly all cases, 

 which will therefore be considered first. 



Nuclear division 



Between a simple direct division on the one hand and a com- 

 plicated indirect division which is comparable with the typical 

 metazoan mitosis on the other hand, all types of nuclear division 

 occur. 



Direct nuclear division. Although not so widely found as it was 

 thought to be in former years, amitosis occurs normally and regu- 

 larly in many forms. While the micronuclear division of the Cilio- 

 phora is mitotic (p. 165), the macronuclear division is invariably 

 amitosis. The sole exception to this general statement appears to be 

 the so-called promitosis reported by Ivanic (1938) in the macro- 

 nucleus in the "Vermehrungsruhe" stage of Chilodenella uncinata in 

 which chromosomes and spindle-fibers were observed. In Para- 

 mecium caudatum (Fig. 52), the micronucleus initiates the division 

 by mitosis and the macronucleus elongates itself without any visible 

 changes in its internal structure. The elongated nucleus becomes 

 constricted through the middle and two daughter nuclei are pro- 

 duced. 



It is assumed that the nuclear components undergo solation during 

 division, since the formed particles of nucleus which are stationary 

 in the resting stage manifest a very active Brownian movement. 

 Furthermore, in some cases the nuclear components may undergo 

 phase reversal, that is to say, the chromatin granules which are dis- 

 persed phase in the non-staining fluid dispersion medium in the rest- 

 ing nucleus, become dispersion medium in which the latter is sus- 

 pended as dispersed phase. By using Feulgen's nucleal reaction, 

 Reichenow (1928) demonstrated this reversal phenomenon in the di- 

 vision of the macronucleus of Chilodonella cucullulus (Fig. 53). 



The macronucleus becomes at the time of its division somewhat 

 enlarged and its chromatin granules are more deeply stained than 

 before. But chromosomes which characterize the mitotic division 

 are entirely absent, although in a few forms in which mating types 

 occur, the type difference and certain other characters, according to 



145 



