26 INTRODUCTION TO CYTOLOGY 



There are usually plastids of one or more kinds in the cytoplasm, 

 the most conspicuous in plant cells being the green chloroplasts. 



A centrosome is present in the majority of animal cells and in those of 

 certain lower plants. It may occupy the center of a visibly differentiated 

 region, the centrosphere or attraction sphere, and at the time of cell- 

 division is the focus of a conspicuous system of radiating astral rays, 

 collectively known as the aster. 



Chondriosomes have now been demonstrated in the cells of nearly all 

 plant and animal groups. These are minute bodies having the form of 

 granules, rods, or threads, and apparently constitute a group of materials 

 having various functions. 



Metaplasmic inclusions are accumulations of food materials and 

 differentiation products that are relatively passive. These non-proto- 

 plasmic bodies may exist in the form of droplets or crystals, and those 

 which are not transitory or reserve food materials apparently play a very 

 minor role in the life of the cell. 



Strictly speaking, the cell wall as at present understood is not a part 

 of the cell proper, or protoplast, but is rather regarded by many as a 

 secretion of the latter. In many cells, particularly those of animals and 

 the motile cells of algae and flagellates, it may be absent. 



The foregoing is a bare sketch of the general structure of a "typical" 

 cell. It is scarcely necessary to point out that the cell should not be 

 thought of as a static thing with a permanent physical structure: it is 

 rather a dynamic system in a constantly changing state of molecular 

 flux, its constitution at any given moment being dependent upon ante- 

 cedent states and upon environmental conditions. As stated by Moore 

 (1912), "the living cell may be regarded, from the physico-chemical 

 point of view, as a peculiar energy transformer, through which a continu- 

 ally varying flux of energy ceaselessly goes on, and the whole life of the 

 cell is an expression of variations and alterations in rates of flow of 

 energy, and of swings in the balance between various forms of energy." 

 In the words of Harper (1919), the cell is a colloidal system in which the 

 various processes have become progressively localized in certain regions, 

 with the resulting formation of organs, which, with the increasing con- 

 stancy of the processes involved, have come to possess a permanence 

 and individuality of their own. In view of the spatial relationship and 

 definite physiological integration of the various components of the cell, 

 we are to look upon the cell not as a mere mixture of complex substances, 

 but as a definitely organized system. 



The Differentiation of Cells. It is a striking fact that in spite of 

 many minor variations the fundamental structure of the cell is essentially 

 similar in nearly all living organisms, and in all the kinds of tissues 

 which go to make up the body of any one of them. As Harper (1919) 

 remarks, "evolution as we know it has not consisted in the production 



