2 DEFENSIVE FERMENTS OF THE ANIMAL ORGANISM 



absorbed materials from the metabolic by-products, 

 and these again from the secretory or excretory pro- 

 ducts. Absorption and secretion merge in each 

 other. The higher we climb in the scale of organi- 

 zation, particularly in the animal kingdom, the more 

 do we meet with cells which are entrusted with 

 special functions. For instance, we find cells which 

 receive matter from the exterior. Others transform 

 particular compounds into products of a special 

 nature. Others, again, have the function of carrying 

 the final products of metabolism to definite points for 

 excretion. 



A unicellular organism stands constantly in rela- 

 tion with numerous substances of the outer world, 

 which differ from place to place and from time to 

 time.. Some of these it makes use of as nutriment. 

 Others, on the contrary, are entirely useless to the 

 cell in question, while many would cause consider- 

 able harm, if allowed to penetrate its wall cells. To 

 these substances the single cell does not yield itself 

 helplessly, but has at its disposal various arrange- 

 ments for its own defence. It has, in the first place, 

 a cell wall which is impermeable by many substances. 

 Further, the cell, by means of different processes, is 

 capable of altering substances, which may in any way 

 be injurious to it, in such a way that the active group 

 is rendered harmless. Often a simple hydrolytic 

 decomposition is sufficient to deprive the compli- 



