MITOTIC DIVISION OF CELLS 75 



more prominently to the front. Thus Gallardo has suggested 

 that the chromatin substance is charged with negative and the 

 cytoplasmic colloids with positive electricity, while the centro- 

 sornes are capable of acquiring a positive potential higher than 

 that of the general cytoplasm. Increase of this potential causes 

 the centrosome to divide and the radiations which form the 

 asters and spindle indicate lines of force in the cytoplasm. The 

 two daughter centrosomes, inasmuch as they bear like charges of 

 electricity, repel one another. In a similar way the chromosomes 

 divide under the influence of their high negative charges and 

 the two halves of each repel one .another and are at the same 

 time attracted by the positive centrosomes. The two new groups 

 of negatively charged chromosomes then attract the positive 

 cytoplasm in opposite directions and thus the division of the cell 

 body follows upon that of the nucleus. 



Whatever may be the physical explanation of these complex 

 phenomena, we must think of them as lying at the root of all 

 normal processes of growth and multiplication in the higher 

 plants and animals. With comparatively rare exceptions, some 

 of which will be mentioned later on, every one of the innumer- 

 able series of cell-divisions initiated by the fertilized ovum, 

 and continued throughout life in the growth and repair of tissues, 

 is accompanied by complicated processes similar to those above 

 described. The process of cell-multiplication, however, is 

 frequently confined in adult organisms to certain regions. 

 Thus, as we have already seen, in the higher animals the growth 

 of the epidermis depends upon cell-divisions which go on only in 

 its deepest layer, the stratum Malpighii (Fig. 18, s.m.). Most of 

 the cells in the body sooner or later lose the power of division, 

 but they are then usually short-lived, as in the case of those 

 cells which form the outer layers of the epidermis and which 

 rapidly become converted into more or less horny scales to be 

 cast off on reaching the surface. 



The majority of the tissues are thus renewed throughout 

 life by the mitotic activity of some unspecialized cell-group, a 

 high degree of specialization in the tissue cells of the higher 

 organisms being always, as we have already seen in the case of 

 red blood corpuscles and nerve cells, accompanied by the loss of 

 the power of multiplication. 



The limitation of cell-multiplication to definite circumscribed 

 regions of the body is perhaps best seen in the case of the higher 



