VITAL MANIFESTATIONS. 5 



guished from the surrounding granules. Its usual position is within the 

 cytoplasm, but the exact location seems to depend upon the focus of 

 greatest cellular activity; thus, in a dividing element, the centrosome lies 

 immediately related to the actively changing nucleus, while within ciliated 

 epithelium it is found closely associated with the contractile filaments con- 

 nected with the hair-like appendages. In recognition of the intimate 

 relations between this minute body and the active changes affecting the cell, 

 the centrosome may be regarded physiologically as its dynamic centre. 

 The centrosome, often represented by a pair of minute granules (diplosome), 

 is frequently surrounded by a clear area or halo, the centrosphere. As seen 

 in certain invertebrate cells, the centrosome is resolvable into a minute 

 central granule, the centriole, embedded within a substance known as the 

 centroplasm. 



The manifestations of the life of the cell include the complex changes, 

 which occur during the performance of its appointed work. The phenomena 

 called "Hfe" probably result from the exercise of the same physico-chemical 

 laws that govern non-living matter; hence, the existence of a "vital principle," 

 long assumed as necessarily present, is discredited by many modern biologists. 

 Such phenomena embrace metabolism, growth, reproduction and irritability. 



Metabolism, the most distinctive characteristic of living matter, is that 

 process whereby protoplasm selects from the heterogeneous materials of food 

 those particular substances which are suitable for its nutrition and converts 

 them into its own substance. Metabolism is of two kinds constructive and 

 destructive. Constructive metabolism, or anabolism, is the process by which 

 the cell converts the simpler compounds into organic substances of great 

 chemical complexity. Destructive metabolism, or katabolism, is the process 

 by which the cell breaks up the complex substances resulting from construc- 

 tive metabolism into simpler compounds. Vegetal cells possess the power 

 of constructive metabolism in a conspicuous degree and from the simpler 

 substances, such as water, carbon dioxide and inorganic salts, prepare food- 

 material for the nutritive and katabolic processes which especially distinguish 

 animal cells. The latter are dependent, directly or indirectly, upon the veg- 

 etal cells for their nutritive materials. 



Growth, the natural sequel of the nutritive changes effected by metab- 

 olism, may be unrestricted and equal in all directions, resulting in uniform 

 expansion of the spherical cell, as illustrated in the growth of the ovum in 

 attaining its mature condition. Such unrestricted growth, however, is 

 exceptional, since cells are usually more or less intimately related to other 

 structural elements by which their increase in size is modified so as to be 

 limited to certain directions. Such limitation and influence result in unequal 

 growth, a force of great potency in bringing about the differentiation and 

 specialization of cells, and, secondarily, of organs and entire parts of the.body. 

 Familiar examples of the results of unequal growth are seen in the columnar 

 cells of epithelium, the fibres of muscular tissue, and the neurones of the 

 nervous system. 



Reproduction may be regarded as the culminating vital manifestation 

 in the life-cycle of the cell, since by this process the parent cell surrenders 

 its individuality and continues its life in the existence of its offspring. Cell- 

 reproduction occurs by two methods the indirect or mitotic and the direct 

 or amitotic. The first of these, involving the complicated cycle of nuclear 

 changes known as mitosis or karyokinesis, is the usual method; the second 

 and simpler process of direct division is exceptional and frequently associated 

 with conditions of impaired vital vigor. 



