DEFENSIVE FERMENTS OF THE ANIMAL ORGANISM 9 



generally takes place in such a manner that the sub- 

 strate is decomposed into simpler indifferent con- 

 stituents, after which the cell builds from the base 

 upwards. 1 '* In many cases this rebuilding is unneces- 

 sary. Such is the case when the cell only requires the 

 energy contained in the absorbed substance. As 

 soon, however, as substances are required as vital 

 units of the cell, then they have to be adapted to 

 the whole structural plan in all its minutest details. 

 This is also the case when secretory substances, 

 having a characteristic structure and a specific action, 

 are to be formed. 



\Ye know of unicellular organisms which produce 

 their body substance from very simple elements 

 indeed. Thus, we know of organisms which 

 produce their cell plasma from carbonates, nitrates, 

 water, and salts. Others can draw their nitrogenous 

 supply from any substance which will supply them 

 with ammonia. Others, again, make use of the free 

 nitrogen of the air. There are, however, even 

 amongst unicellular organisms, some that are very 

 fastidious and will only thrive in the presence of 

 certain peptones. Others even require certain forms 

 of protein from which to obtain their derivatives. 



An exhaustive study of the sources of nitrogen 



1 a See in connection with this, Emil Abderhalden, " Syn- 

 these der Zellbausteine in Pflanze und Tier," Julius Springer. 

 Berlin, 1912. 



