26 THE CELL 



tain Ca salts which have so favorable an influence in the higher animals, are 

 poisonous for the Protista, while other Ca salts are harmless for them. 



Besides the elements just mentioned (Na, Ca, K and Cl) and those con- 

 tained in the proteids, fats and carbohydrates (C, H, O, N, S, P), there are 

 still a few others which are just as necessary for the animal body. First among 

 these are : Mg, contained with the Ca in the solid framework of the bones ; Fe, 

 which is necessary for the formation of the coloring matter in the red cor- 

 puscles; and I, which is a necessary constituent of the secretion of the thyroid 

 gland. 



In all processes of assimilation in nature, whatever their kind, energy is 

 stored up. As a measure of the energy contained in a substance we use the 

 amount of heat developed by its combustion. Carbon dioxide and water are 

 not combustible substances, but starch produced from them generates a con- 

 siderable quantity of heat when it is burned possesses therefore a certain 

 quantity of potential energy, amounting in fact to about 4.1 Cal. 1 per gram. 

 This potential energy is derived from the sunlight, whose kinetic energy has 

 been transformed under the influence of chlorophyll to the potential energy 

 of starch. 



When a synthesis takes place in a living cell, if the necessary energy is not 

 supplied from without as in starch formation, the synthesis can only be 

 carried out at the expense of potential energy stored in the cell itself. In other 

 words : in all synthetic processes taking place in plant or animal cells without 

 the agency of sunlight, the potential energy at the disposal of the cell is 

 transformed in one way or another into the potential energy of the newly 

 formed substance. 



The assimilative functions of cells are closely bound up with dissimilative 

 functions i. e., if the cell has not the power to develop kinetic energy within 

 itself no new formation of substance appears to take place and conversely, 

 the more rapid the dissimilative process, the more active is the assimilative 

 process. Neither plants nor the eggs of animals can develop continuously without 

 oxygen, as was determined successively by Spallanzani, Dutrochet, De Saussure 

 and Schwann. In the egg of Ctenolabrus, a marine bony fish, the cleavage cells 

 undergo partial solution and fuse together when oxygen is withdrawn, but are 

 reformed when oxygen is again supplied (Loeb). Possibly in this connection 

 belong also the facts: that growth is always accompanied by dissimilation, and 

 that skeletal muscles increase in size only under the influence of work (involving 

 dissimilation, cf. page 63). 



B. The dissimilative processes constitute the source of the kinetic energy 

 developed in the cells. These processes in plant as well as in animal cells are 

 everywhere essentially similar, and consist in a destruction of complex mole- 

 cules. Whether this destruction involves the living substance of the cell, or 

 only the nonliving cell contents cannot yet be definitely decided. Under the 

 general subject of metabolism we shall find opportunity to discuss this question 

 somewhat more exhaustively. Here we must limit ourselves to the following : 



1 1 Cal. (Calorie) = the amount of heat necessary to raise 1 kg. of water from 

 to 1 C. 



