246 SPH^RELLA CHAP. 



shown that this oxygen is produced by the decomposition 

 of the carbon dioxide contained in solution in rain-water, 

 and indeed in all water exposed to the air ; the gas, which is 

 always present in small quantities in the atmosphere, being 

 very soluble in water. 



As the carbon dioxide is decomposed in this way, its 

 oxygen being given off, it is evident that its carbon must be 

 retained. As a matter of fact it is retained by the organism 

 but not in the form of carbon ; in all probability a double 

 decomposition takes place between the carbon dioxide 

 absorbed and the water contained in its protoplasm, the 

 result being the liberation of oxygen in the form of gas and 

 the simultaneous production of some extremely simple 

 form of carbohydrate, i.e., some compound of carbon, 

 hydrogen, and oxygen with a comparatively small number 

 of atoms to the molecule. 



The next step seems to be that the carbohydrate thus 

 formed unites with the ammonia salts or the nitrates absorbed 

 from the surrounding water, the result being the formation 

 of some comparatively simple nitrogenous compound. 

 Then further combinations take place, substances of greater 

 and greater complexity are produced, sulphur from the 

 absorbed sulphates enters into combination, and proteids 

 are formed. From these, finally, fresh living protoplasm 

 arises. 



From the foregoing account, which only aims at giving 

 the very briefest outline of a subject as yet imperfectly 

 understood, it will be seen that, as in Amoeba, the final result 

 of the nutritive process is the manufacture of protoplasm, 

 and that this result is attained by the formation of various 

 substances of increasing complexity. But it must be noted 

 that the steps in this process of constructive metabolism 

 are widely different in the two cases. In Amoeba we start 



