Page:Defensive Ferments of the Animal Organism (3rd edition).djvu/29

 and the blood. Finally, they may be easily withdrawn from the body before they have had any opportunity of penetrating into the interior of the cells.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) 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 decomposed 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 they probably never break down such complicated substances 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