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TEE POPULAR SCIENCE MONTHLY 



Memb 



-© 



B 



Fig. 1. A nearly ripe human ovum in the living condition. The ovum is sur- 

 rounded by a series of follicle cells (FG) inside of which is the clear membrane 

 (Memb.) and within this is the ovum proper containing yolk granules (Y) and a 

 nucleus (N) embedded in a clear mass of protoplasm. Magnified 500 diameters 

 (x 500). (From O. Hertwig.) B, two human spermatozoa drawn to about the same 

 scale of magnification. (After G. Ketzius). 



or less from one another, which are bound together and integrated into 

 a single organism. Living cells and organisms are not static structures 

 which are fixed and stable in character, but they are systems which are 

 undergoing continual change. They are like the river, or the whirlpool, 

 or the flame, which are never at two consecutive moments composed of 

 the same particles but which nevertheless maintain a constant general 

 appearance ; in short they are complex systems in dynamical equilibrium. 

 The principal physiological processes by which all living things 

 maintain this equilibrium are: 



2. Metabolism, or the transformation of matter and energy within 

 the living thing, in the course of which some substances are oxidized 

 into waste products, with the liberation of energy, while other sub- 

 stances are built up into protoplasm, each part of the cell converting 

 food substances into its own particular substance by the process of 

 assimilation. 



3. Reproduction, or the capacity of organisms to give rise to new- 

 organisms, of cells to give rise to other cells, and of parts of cells to give 

 rise to similar parts by the process of division. 



4 Irritability, or the capacity of receiving and responding to im- 



