82 BIOLOGY. [Book ii. 



the enveloping membrane ; they are modified and renewed un- 

 ceasingly, molecule by molecule, without being destroyed. 



In the primordial phenomena of nutrition there are, in effect, 

 two acts, or rather two principal aspects of the same phenomenon, 

 assimilation and disassimilation. To assimilation relate the facts 

 of absorption and endosmosis ; with disassimilation are connected 

 the facts of secretion and exosmosis. We must remember that 

 disassimilation has, as result, the transformation of the colloidal 

 substances of living bodies into crystallisable substances, occupy- 

 ing, after a fashion, a middle position between organic substances 

 and mineral substances. 



It is now invincibly demonstrated that these primordial facts 

 of nutrition are identical in all the living universe, as well in 

 the animal world as in the vegetal world. It is also known, 

 moreover, that the principal agent of all these transformations, 

 of all these exchanges, is the oxygen of the air. 



In the most rudimentary beings, amorphous or monocellular, 

 oxygen is diffused direct in the midst of the molecules of the 

 living substance ; it oxydises this substance by a sort of slow 

 combustion, and determines the formation of diverse organic 

 crystallisable bodies, and of a gas — carbonic acid ; the whole is 

 afterwards expulsed. 



In the being whose structure is more complex, where there is 

 an aggregation of cells, of fibres, in a word, of diverse histological 

 elements, each having its special form and special functions, 

 while the whole are, moreover, grouped in particular tissues, the 

 oxygen of the air, and generally all the substances which penetrate 

 into the organism and come forth from it, have to undergo a sort 

 of gradatory process before being assimilated or excreted. In 

 the simplest cases when the organism is merely constituted by 

 histological elements of kindred nature, more or less straitly 

 joined together, and bathing in an interstitial liquid, a circulatory 

 system, a respiratory system, a digestive system being wholly 

 lacking, the nutritive substances and the disassimilated substances 

 dissociate themselves in the intercellular blastema. It is in this 



