546 GENERAL PHYSIOLOGY 



possessing nucleus and protoplasm does not always mediate 

 hereditary transmission. 



To summarise, the character of every cell is determined by its 

 peculiar metabolism. Hence, if the peculiarities of a cell are to 

 be transmitted, its characteristic metabolism must be transmitted ; 

 this is only conceivable when nuclear substance and proto- 

 plasm with their metabolic relations are transferred to the 

 daughter-cells. This is true of the sexual reproduction of the 

 higher animals, as well as of the asexual reproduction of unicel- 

 lular organisms; in the former, however, the metabolism of one cell, 

 the spermatozoon, is by the process of fertilisation combined with 

 that of another cell, the ovum, into a single resultant, the metabolism 

 of theoffspring that arises from the fertilised ovum ; the offspring 

 hence possesses the characters of the two parents. 



3. The Mechanics of the Transformation of Energy hy the Cell 



The third aspect in which the changes of a body make them- 

 selves manifest, besides those of substance and of form, is that of 

 transformation of energy. The three aspects are inseparable and 

 are the expression of all that happens in the physical world. 

 Given one of the three in all its details, the other two would 

 be known. This is true of both living and lifeless bodies, for both 

 are physical systems and must obey the strict laws of all matter. 



a. The Circulation of Energy in the Organic World 



Unfortunately, as regards the transformation of energy in the 

 living organism, our knowledge at the present time is but 

 fragmentary. The beginning and the end are known, but between 

 the two is the complex series of events in which the energy in its 

 passage through the living substance takes part, and thus far only 

 a few of these events have been discovered. But so much is 

 evident : the changes of energy are just as manifold in their 

 details as are the changes of substance and of form, and every 

 kind of cell is characterised as much by the former as by the 

 latter. It has been seen that the green plant-cell is that form of 

 living substance which in a certain sense is the basis of all 

 life now existing upon the earth's surface, in so far as it is the 

 laboratory in which from inorganic materials organic compounds, 

 which are the necessary vital condition of all other organisms, are 

 manufactured. Hence, in outlining a scheme of the general circu- 

 lation of energy in living nature, attention must be given to the 

 green plant as the starting-point for the entrance of energy into 

 the living physical world. 



That form in which energy is introduced into the green plant- 



