HYPERTEOPHY, HYPERPLASIA, METAPLASIA, REGENERATION. 95 



We shall now briefly summarize the morphological changes which 

 cells undergo in division and shall then indicate the degree of regener- 

 ative capacity which various forms of tissues possess. 



MODES OF CELL DIVISION. 



The careful and minute study of cells during the act of division, 

 which has been recently made, has revealed many most curious phe- 

 nomena and has opened a new world of observation nearer to the elemen- 

 tary expression of life than has seemed possible in earlier times. It will 

 suffice for our purposes briefly to indicate some of the more striking feat- 

 ures of the new cell lore. 



It is well to recall at the outset that recent studies of cells have shown 

 that even in their simplest forms they are highly organized, and that 

 their different parts have special functions to perform. Thus the nucleus 

 presides over the constructive metabolism or assimilative process of the 

 cell and furnishes the physical basis upon which the transmission of 

 hereditary characters depends. The cytoplasm of the body, on the other 

 hand, is concerned in those phases of metabolism which result in the 

 liberation of energy in movements of various kinds and in the formation 

 of new chemical substances. The centrosome also in certain cells, though 

 not apparently in all, appears to play an important part in the changes 

 incident to division. 



Two modes of cell division are commonly recognized. First, indirect 

 division (mitosis, or karyokiuesis ) ; second, direct division (amitosis). 



INDIRECT (MITOTIC) CELL DIVISION. 



This is the most common mode of cell division and is especially char- 

 acteristic of embryonic cells and those which are undergoing active de- 

 velopment. While it presents great variations, its general features may 

 be thus briefly summarized : 



Among the earlier changes which are to be seen in a cell about to 

 divide by mitosis are a condensation and an increase in the staining capac- 

 ity of the chromatiu of the intranuclear network. This chroinatin sub- 

 stance gathers into a contorted thread or threads, called the spircim 1 * 

 (Fig. 30, 2), within the nucleus, whose membrane with the uucleolus 

 gradually disappears so that the spireme lies free in the cytoplasm ; and 

 at the same time with, or preceding, these changes in the nucleus, there 

 may be a division of the centrosome when this is present, the segments 

 resulting from this division passing to opposite parts of the cell, usually 

 outside the limits of the nucleus. Around each of the new centrosomes, 

 which stain deeply with hsematoxylin or other nuclear dyes, may be a clear 

 zone of unstained material, or a series of fine radiating fibrils, or both ; 

 the whole forming a structure called a polar body (Fig. 30, 2 and 3). 



Xow the threads of the spireme break across transversely, forming a 

 series of more or less rod-like bodies called chromosomes, which form a 



