4 DEFENSIVE FERMENTS OF THE ANIMAL ORGANISM 



and the blood. Finally, they may be easily with- 

 drawn from the body before they have had any 

 opportunity of penetrating into the interior of the 

 cells. 



As a principal defence a single cell always has the . 

 cell wall, with its characteristic construction and its 

 specific physical properties. Besides this, there is no 

 doubt that ferments play a considerable role. They 

 allow the cell to make a choice from amongst the 

 substances which are continually acting upon it. 

 These ferments, as Emil Fischer (Lit. 6) 1 has proved 

 from his exact researches on the subject, are directed 

 in a specific manner against definite substrates. Only 

 those substances, which are capable of being decom- 

 posed by the cell into simpler groups, are in general 

 found to be of use to it. Throughout, our positive 

 knowledge has led us to the conclusion that cells 

 supply their vital needs only from the simplest 

 units of the nutritive material, and that thev 



j 



probably never break down such complicated sub- 

 stances as fats, polysaccharides, and proteins, directly 

 into their final metabolic products. Even the 

 simplest units are not at once completely broken 

 up. The cell works in stages. First of all it splits 

 up large molecules into smaller particles, and 

 so sets free from the rest one fraction of the entire 



1 The numbers refer to the Bibliography given at the end 

 of the book. 



