28 MORPHOLOGY AND CULTURE OF MICROORGANISMS 



The nuclear phenomena are much more important, and better 

 known. The nucleus divides in order to furnish each daughter cell 

 with a nucleus containing the same amount of chromatin. 



NUCLEAR DIVISION. Nuclear division may occur in one of two 

 ways, one very complex, (i) the indirect mode, karyokinesis or mitosis; 

 the other very simple, (2) the direct mode, or amitosis. 



Indirect Division, Karyokinesis, or Mitosis. We shall begin with 

 the indirect mode which is by far the more common, using as an example 

 a Heliozoon, the Acanthocystis aculeata (Fig. 20, A). The nucleus of 

 this protozoon at rest contains a large karyosome of a spongy structure, 

 and a chromatic network. Outside the karyosome in the nuclear 

 vesicle is a centriole surrounded by a hyaline zone, the archoplasm 

 (Fig. 20, A, a). 



Mitosis may be divided into four steps or phases. 



The first phase or prophase begins by the emigration of the centriole 

 from the nucleus outside of which it surrounds itself by cytoplasmic 

 irradiations, making a star-like body, called the aster (Fig. 20, A, b). 

 Following this, the karyosome dissolves in the nucleoplasm, supposedly 

 conveying material to the chromatic network which enriches itself 

 noticeably in chromatin. The chromatic network then relaxes, thickens 

 and transforms itself into a more or less spiral cluster, the spireme 

 (Fig. 20, A , c) . At the same time the centriole divides into two centrioles, 

 each surrounded by an aster (Fig. 20, A, c). Soon these centrioles place 

 themselves at the two opposite poles of the nucleus (Fig. 20, A, d), while 

 the spireme breaks itself up into a definite number of chromatic sec- 

 tions, the chromosomes. While this is taking place, the nuclear mem- 

 brane dissolves itself into a series of cytoplasmic fibrils, the achromatic 

 spindle, resistant to nuclear stains. They appear in the middle of 

 the nucleus and converge at each end to the centrioles (Fig. 20, A, d, 

 c). The chromosomes group themselves in the center of the spindle 

 as the equatorial plate (Fig. 20, A, e), the formation of which completes 

 the prophase. Each of the chromosomes is attached to one of the 

 fibrils which make up the achromatic spindle. 



The second phase or metaphase consists of the longitudinal di- 

 vision of the chromosomes each of which divides itself into two equal 

 chromosomes. 



In the third phase or anaphase the chromosomes equally divided 



