THE CELL AND 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 io the daughter cells. 



PRACTICAL SUGGESTIONS. 



The larval salamander and newt are classical subjects for the study of cell division. 

 The small larvae are fixed in Flemming's fluid, cut into thin sections in either celloidin or par- 

 affin and stained with Heidenhain's haematoxylin (see Appendix). Mitotic figures may be 

 found in almost any of the tissues. 



Certain vegetable tissues, such as magnolia buds or the root-tips of rapidly growing 

 onions, also afford excellent material for the study of mitosis. The technic is the same as 

 for animal tissues. 



References for Further Study. 



Conklin, E. G.: Karyokinesis and Cytokinesis. Jour. Acad. Nat. Sci. of Philadel- 

 phia, Vol. XII, 1902. 



Heetwig, O.: Die Zelle und die Gewebe. 1898. 



Lillie, F. R.: A Contribution towards an Experimental Analysis of the Karyokinetic 

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



Wilson, E. B.: The Cell in Development and Inheritance. 2d Ed., 1900. 



