34 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



name of mesomitosis has been given to all the mitoses which distinguish 

 themselves from promitosis by the persistence of a nuclear membrane 

 throughout the phenomenon. 



Direct Division or Amitosis. — This consists simply of an elongation 

 of the nucleus followed by a median constriction, then by a rupture of 

 this constricted part without an equal division of the chromatin be- 

 tween the two nuclei which often are not the same size. It is a simple 

 breaking up of the nucleus. Amitosis, then, does not necessarily in- 

 sure the equal distribution of chromatin between the two nuclei. 

 This rare process is found in higher organisms only in old cells that are 

 degenerating, or in diseased cells. Although for a long time it "was 

 thought to be a primitive phenomenon, it is now considered to be 

 degenerative. We see, however, in certain Amoeba and Mycetozoa 

 the karyosome enclosing all the chromatin divides itself into two equal 



Fig. 22. — Conjugation in Schizosaccharomyces octosporus. (a) Two gametes in 

 the process of fusion; (6) (c) nuclear fusion. 



bodies, showing the characteristics of a very primitive mitosis (Fig. 17). 

 Amitosis seems to exist normally in yeasts and in certain molds. In 

 the yeasts, for example, the nucleus divides by amitosis in the course 

 of budding (Fig. 2), and mitosis is found only in the course of sporu- 

 lation. 



Sexual Changes. — In most microorganisms at certain times during 

 their existence occur sexual changes, or fertilization, which seem to give 

 them a new strength. It is followed by a period of very active re- 

 production, whence the name of sexual reproduction given to these 

 changes, This consists essentially in the fusion of two equal 

 isogamous (isogamy) or unequal, anisogamous {heterogamy) cells or 

 gametes. In the latter case, the male is small and active, and the 

 female large and passive. The fusion between the two cytoplasms 

 and the two iiuclei takes place at the same time (Fig. 22). 



If nuclear fusion were not compensated by an elimination of 

 chromatin, the nucleus would increase in this substance at each fertih- 

 zation. But this change is succeeded immediately in protozoa by a 



