112 



INTRODUCTION TO CYTOLOGY 



that these approximate the expectations based on Van't Hoff's law for 

 the velocity of chemical reactions. 



When cytokinesis follows mitosis immediately, as it does in most 

 tissues, it is carried through with considerable rapidity. In higher 

 plants the cell-plate commonly appears during the closing phases of 

 mitosis and by the time the daughter nuclei are fully reorganized the 

 new cell membrane is well formed. In accelerated motion pictures of 

 living fibroblasts and animal eggs the cells, after a series of interesting 

 movements which escape the observer of fixed material, constrict and 

 complete their division with dramatic suddenness. 



In root tips, which are favorite objects for the study of somatic 

 mitosis in plants, several investigators'* have found a periodicity in the 

 occurrence of cell-division. Although their results are not in close 



« 



« 





Fig. 59. — Mitosis in a living Chorthippus lineatus spermatocyte in lA'' Ringer solution. 

 The change from 1 to 2 occupied 13 minutes; from 2 to 3, 21 minutes; from 3 to 4, 46 min- 

 utes. {After Belar, 1929a.) 



agreement, they indicate that there are often about two division maxima 

 during a 24-hour period, one of these falling near the middle of the 

 day, and also that the rate of root elongation tends to be highest at the 

 time of minimum cell-division. Many experiments have shown that 

 while the periodicity in such cases may be altered by changing the 

 environmental conditions, there is a noticeable tendency to retain the 

 habitual rhythm. This indicates the action of both external and inherent 

 factors, whose respective roles in determining the observed results have 

 not been fully analyzed. 



Summary and Conclusions. — Typical somatic cell-division includes 

 both mitosis and cytokinesis. In the prophase of mitosis the nuclear 

 reticulum is resolved into a number of threads; these threads, or chro- 

 monemata, are the chromosomal elements which together formed the 

 reticulum in the preceding telophase. They are double as the result of 

 an accurate longitudinal splitting. A less highly stainable chromosomal 

 matrix becomes evident about the chromonemata. In the metaphase 



^Kellicott (1904), Karsten (1915, 1918), Friesner (1919, 1920), StSJfelt (1919, 

 1921), Fortuin (1926), Kojima (1928). 



