50 THE CELL 



double, this being apparently in preparation for the succeeding cell- 

 division. In some cases the centrosome is triple or even multiple. 

 It was first found in the ovum and described as peculiar to that 

 cell. It is now believed to occur in most, if not in all, animal 

 cells. It usually consists of (i) a minute central granule or granules — 

 the centriole, which stains intensely with iron-haematoxylin, and out- 

 side of which is (2) a clear zone, the attraction sphere. From this 



centre, radiations extend outward into the 

 cytoplasm. There is much confusion of 

 terms in connection with the centrosome, 

 the term centrosome being by some appHed 

 to the entire structure including the radiat- 

 ing fibrils, by others to the central granule 

 only, by still others to the central granule 

 plus the surrounding clear area. By some 

 -r,^ c . the radiations are believed to be composed 



riG. 5. — bpermatogo- '■ 



nium from frog (Hermann), of a different substance than the general cy to- 

 Single centrosome at cen- , ,.,.,. , , 7,7 ^1 

 ter of attraction sphere or piasm, which IS designated arcJio plasm. The 



aster. Nucleus contains a main significance of the centrosome is in 

 plasmosome. . . 



connection with cell-division, under which 



head it will be further considered (p. 53). 



Vital Properties of Cells 



It has already been noted that the essential peculiarity of the 

 cell is that it possesses certain properties which are characteristic 

 of life. By this is meant that a cell is able: i. To nourish itself and 

 to grow — metabolism. 2. To do its own particular work in the body 

 economy — special function. 3. To respond to stimulation — irritability. 

 4. To move — motion. 5. To produce other cells — reproduction. 



As would be expected, these properties, existing as they do in 

 living cells, cannot always be sharply separated but frequently over- 

 lap. Thus, e.g., in the muscle cell "motion" equals "special func- 

 tion." 



In the simplest forms of animal life, where a single cell constitutes 

 the entire individual, all of these functions are performed by the one 

 cell. In all higher, that is, multicellular animals, there are not only 

 many cells but many kinds of cells, and this morphological differen- 

 tiation corresponds to a physiological differentiation, each group of 

 cells developing along certain well-defined lines for the performance 

 of its own special function. 



