THE MITOCHONDRIAL CONSTITUENTS OF PROTOPLASM. 129 



X. PHYSIOLOGY. 



There is a very widespread belief that mitochondria play an active part in 

 cellular acti\dty, though the nature of their behavior is almost wholly obscure. 

 We must approach a problem so difficult with caution and take stock of the possi- 

 bilities before we attempt to arrive at any conclusion. Now, vital processes, or 

 life phenomena in the cells, which are the ultimate structural units m our bodies, 

 are naturally divided into two great .groups — those which are fundamental, being 

 common to all cells, and those which are special, representing the peculiar duties 

 which certain highly differentiated, older cells have learned how to perform in the 

 course of their development. 



Among the fundamental activities of protoplasm we are accustomed to group 

 metabolism, respiration, irritability, growth, and reproduction. Young cells pos- 

 sess all of them as contrasted with the older, more mature cells of later stages. 

 These older, fully differentiated cells can no longer reproduce their kind, but never- 

 theless contain mitochondria, so that we may safely eliminate reproduction from 

 consideration. Irritability is the power of being able to respond in some way to 

 variations of any kind in the environment. The stimulus varies, but the mitochon- 

 dria are constant. Thej^ are no more numerous in nerve-cells in which irritability 

 is developed to an extraordinary degree than in other cells. This leaves for con- 

 sideration only metabolism and respiration (which is really a special phase of 

 metabolism), though some would be inclined to include the "mnemonic factor" 

 with these basic activities of life. Mathews (1915, pp. 68 and 587) refers "memory" 

 to the cells themselves, and gives us an inkling of the possible chemical reactions 

 upon which it may be based, though psychologists tell us that it is due to the 

 interaction of reflex arcs rather than being a manifestation of the life of individual 

 cells. It is highly probable that mitochondria are concerned in growth. 



The differential activities, on the other hand, are secretion in gland-cells, 

 contraction in muscle-cells and ciUa, and irritability and conduction in nerve-cells. 

 As a matter of fact they are merely the fundamental activities of living material, 

 enhanced and intensified, for we must admit that even the most embryonic cells 

 are capable of giving off substances, that is to say, of secretion. 



SECRETION. 



Ever since the work of Altmann there has been a tendency to assume that the 

 mitochondria play a part in the formation of secretion granules. This tendency 

 blossomed out vigorously under the influence of the more recent theoretical con- 

 siderations of Meves and Regaud. Meves (1908, p. 845) thought that the mito- 

 chondria constituted, in part, the material basis of heredity and he believed that 

 they become chemically transformed into all kinds of cellular differentiations like 

 secretion granules, fibrils, etc. The working hypothesis of Regaud (1911, p. 685) 

 was a little different. He at once perceived the difficulty of assuming that the 

 phosphatid albumin mitochondrial complex becomes chemically altered into a 

 large variety of different materials and advanced his "eclectosome " theory, 



