absorbed materials from the metabolic by-products, and these again from the secretory or excretory products. Absorption and secretion merge in each other. The higher we climb in the scale of organization, 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 relation 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 considerable 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 arrangements 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-
