40 NORMAL HISTOLOGY. 



esses together constitute metabolism, the former being constructive 

 metabolism and the latter destructive metabolism. 



The cell, by the rigidity of its walls or its contents, gives support 

 to the structures of the body. 



The cell often serves as a reservoir for the products of assimila- 

 tion. 



The cell is the agent by which new cells are formed and by which 

 the plant or animal is reproduced. 



The method by which new cells are formed is a process of divi- 

 sion. There are four forms of cell division viz., normal fission, 

 budding, free cell formation, and karyokinesis. 



Normal fission is the simple division of a cell in which the proto- 

 plasm divides and a partition is formed between the two halves. 

 Sometimes it takes place by a constriction of the cell-wall. 



Budding. This consists in the formation of a rounded projec- 

 tion, or bud, on the wall of the parent cell. This bud develops to 

 normal size, becomes cut off by a partition, and generally separates 

 from the original cells ; example, yeast. 



Free cell formation takes place when the protoplasm of the cell 

 separates into one or more distinct masses, each mass forming for 

 itself a cell-wall. The new cells are finally set free by the bursting 

 of the wall of the parent cell. 



Karyokinesis. This is a form of fission in which the cell under- 

 goes a cycle of changes, eventually producing two cells from one. It 

 is more common among animals than plants. 



For the study of karyokinesis the growing- tips of onions and the 

 larvae of salamanders may be used. The cells may be fixed with Flem- 

 ming's solution or chromic acid. Stain by the usual methods. 



The different stages through which the cell passes in karyoki- 

 nesis are : "Resting nucleus, the skein, the rosette, the aster, the di- 

 aster, daughter rosettes, daughter skeins, and daughter nuclei. 



REPRODUCTION. 



Plants and animals reproduce asexually and sexually. 



(1) Asexual reproduction. This method is usually accom- 

 plished by the individual cell, and there are, therefore, no distinc- 

 tions of sex, the new cells formed being exactly like the parent cell. 



