THE CELL AXD CELL PROLIFERATION. 9 



The process usually requires from one- half to three-quarters of an hour, but 

 may extend over from two to three hours. 



Mitosis is naturally most active wherever active growth of tissue is taking 

 place for example, in embryonic tissues, in granulation tissue, in the healing 

 of wounds, in rapidly growing tumors (usually an evidence of malignancy). 

 The earlier generations of cells derived from the fertilized ovum are indifferent 

 cells in the sense that they are capable of development into any type of tissue 

 cells. As differentiation takes place, the cells assume more definite and fixed 

 types. With differentiation, mitosis becomes less and less active and cells 

 become incapable of producing cells of any type other than their own. Finally, 

 the most highly differentiated (specialized) cells for example, muscle cells and 

 nerve cells lose entirely their powers of reproduction, and if destroyed are not 

 replaced by new cells of the same type. 



What is known as multipolar or pluripolar mitosis occurs in some of the 

 higher plants, less commonly in the rapidly growing connective tissue of healing 

 wounds and in cancer cells. Such atypical mitosis has also been artificially 

 induced in rapidly dividing cells by the injection of chemical substances into the 

 tissues. In multipolar mitosis the centrosome divides into more than two 

 daughter centrosomes and not infrequently results in an unequal distribution, of 

 chromatin to the daughter cells. 



References for Further Study. 



BUCHXER, P.: Praktikum der Zellenlehre. Erster Teil, Berlin, 1915. 



CONKLIX, E. G.: Karyo kinesis and Cytokinesis. Jour. Acad. Nat. Sci. of Philadel- 

 phia, Vol. XII, 1902. 



HEIDEXHAIX, M.: Plasma und Zelle, Abteilung I, 1907, Abteilung II, 1911. 



HERTWIG, O.: Die Zelle und die Gewebe. 1908. 



KELLICOTT, \V. E.: General Embryology, 1913. 



LILLIE, F. R.: A Contribution towards an Experimental Analysis of the Karyo kinetic 

 Figure. Science, Xew Series, Vol. XXVII, 1908. 



WILSON, E. B.: The Cell in Development and Inheritance. 26. Ed., 1900. 



