8 



CYTOLOGY 



CHAP. 



similar groups, from each of which groups a new nucleus is con- 

 stituted. 



Certain cell structures — the centrosomes and spindle, forming 

 together the achromatic figure — though generally outside the nucleus, 

 are inseparably connected with mitosis and must be considered with it. 



The process of mitosis is illustrated by the diagrammatic Fig. 2, 

 while Figs. 3 and 4 show how the principal stages actually appear under 

 the microscope. Fig. 3 shows a mitosis of a nucleus with abundant 



Fig. 4. 



The first cleavage mitosis in the egg of Echinus esculent its (micro-photographs by Professor T. H. Bryce). 

 A, late prophase, nuclear membrane breaking down ; B, metaphase, C, early, and D, late, anaphase. 



chromatin, but not very voluminous achromatic figure, while Fig. 4 

 represents a mitosis of a nucleus poor in chromatin, but provided with 

 a very well developed achromatic figure. 



The sequence of events in mitosis is commonly divided into four 

 main phases, namely, prophase, metaphase, anaphase (Strasburger, 1884) 

 and telophase (Heidenhain, 1894). It must not be forgotten, however, 

 that these are arbitrary divisions of a continuous process. 



The prophase consists essentially in the reconstruction of the chromatin 

 and Hnin of the resting nucleus into filaments, which by a process of 



