30 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



by a nuclear membrane in which is included the centriole. Thus the 

 two nuclei are formed in which a nucleolus soon appears. Mean- 

 while the cell has elongated, become constricted in the center, and 

 finally broken into two cells (Fig. 20, B, f, g, f). The achromatic 

 spindle completely disappears. 



This method of division represents the typical method of karyo- 

 kinesis, that which is observed in higher organisms with the single 

 difference that the centriole is intranuclear, whereas in the cells of 

 higher organisms it is ordinarily outside the nucleus in contact with the 

 nuclear membrane. An analogous mitosis is found in the Uredinea 

 (Fig. 20, B, a-i), except that the centriole is here found to be extra- 

 nuclear (Fig. 20, B, a), the asters are lacking, and the nucleolus persists 

 to the end of mitosis expelled in the cytoplasm. The physiological 

 significance of the nucleolus in this case is not known. This method of 

 division is seen in certain molds and higher protozoa, and is called 

 metamitosis or perfect mitosis. 



Summing up, mitosis is a process functioning to make an absolutely 

 equal division of the chromatin between the two nuclei. This dis- 

 tribution is performed by the breaking up of a spireme into a definite 

 number of chromosomes, a number varying according to the species 

 but always constant for any single species, and then by a longitudinal 

 division of the latter. The centrioles seem to play an important role 

 in this phenomenon, in directing it, and in attracting the chromosomes 

 once divided toward the poles of the cell where the nuclei are formed. 



It is not necessary to conclude that the processes of mitosis are 

 as complex as in other microorganisms. Relatively simple in the 

 lower forms, mitosis becomes complicated as it climbs the ladder, 

 gaining the characteristics of metamitosis only in the most advanced 

 forms. 



The simplest case is found in the Cyanophycea (Fig. 6). Here 

 cellular division begins by the outline of the transverse partition 

 which appears in the form of a peripheral ring. At the same time 

 the chromatic network takes a definite arrangement; its filaments 

 arrange themselves parallel to the longitudinal axis of the cell, thus 

 giving this division the appearance of a mitotic division. The outline 

 of the partition extends little by little toward the middle of the cell, 

 leaving open only a small spherical space in its center to which the 

 fibers of the network then contract, and the nucleus takes the form of 



