20 INTRODUCTION 



usually appears in each nucleus at the end of the telophase. Also, 

 by this time, the centriole has divided into a diplosome, or 2 cen- 

 trioles, near each daughter nucleus. If the daughter cells chance 

 to be part of a very young embryo, the next cell division will occur 

 very quickly. But if the cells are part of an adult organism, some 

 time may elapse before they divide. 



Amitosis. — Now and then cells appear to divide directly by 

 constriction without the elaborate preparations occurring in mitosis. 

 This process has been named amitosis. In such cases the nucleus 

 first separates into two parts by constriction, after which the cyto- 

 plasm likewise divides. It has been observed in pathological tissues 

 and in the case of some old cells in tissue cultures. In most of the 

 so-called cases of amitosis, the technique may have been at fault 

 or the cells only appear to divide amitotically, due to some abnormal 

 condition interfering with the mitotic process. 



Cell Metabolism.— The living cell is a dynamic system in which 

 chemical and physical changes are constantly taking place. All 

 such transformations of matter and energy occurring in li^'ing proto- 

 plasm are designated by the term metabolism. In this term are 

 included the anabolic processes involved in building tissues up as 

 well as the katabolic or breakdown processes. Nutritive sub- 

 stances taken into the cell are subjected to the action of enzymes 

 which facilitate the breaking down of the complex compounds into 

 their simpler units. These units are then utilized in building up 

 the elements of protoplasm which serve for repair and growth. 

 The energy liberated by oxidation reactions is utilized for the 

 chemical and physical changes involved in movement, in secretion 

 and excretion, and for the continuation of those metabolic changes 

 constantly involved in the maintenance of life. 



All cells regardless of the degree of their functional specialization 

 must still carry on a basic general metabolism necessary to maintain 

 their own life. Thus a muscle cell, in addition to the basic metab- 

 olism maintaining its life, has special metabolic processes endowing 

 it with the property of contraction. 



Cytomorphosis designates the structin-al changes undergone by a 

 cell in its passage from the embryonic state, through its differenti- 

 ated phase to senescence and death. Death or necrosis of cells is 

 followed by cellular disintegration, at which time the enzymes active 

 in the living cell in the synthesis of ])r()teins from amino-acids begin 

 breaking down the proteins into amino-acids which are then diffused 

 from the cell. During this process of autolysis the chromatin of 

 the nucleus contracts into a dense irregular chromatic mass. 



