2. Since chemical action always implies at least two bodies to react, Ehrlich assumes that in every cell which is affected by a chemical stimulus there must therefore be a chemical group to unite with this stimulus. He further states that there must be as many different kinds of these groups as there are different kinds of chemicals which stimulate the cell. Since these groups are present in the body cells to _take up_ different kinds of chemical substances, Ehrlich calls them _receptors_. Since these groups must be small as compared with the cell as a whole, and must be more or less on the surface and unite readily with chemical substances he further speaks of them as “side-chains” after the analogy of compounds of the aromatic series especially. The term _receptors_ is now generally used. As was stated above, the effect of _specific chemical stimuli_ is to cause the production of _more of the particular substance_ for which it is specific and in the class of bodies under discussion, the _particular product is these cell receptors_ with which the chemical may unite. 3. Weigert first called attention to the practically constant phenomenon that cells ordinarily respond by doing more of a particular response than is actually called for by the stimulus, that there is always an “overproduction” of activity. In the case of chemical stimuli this means an _increased production of the specific substance_ over and above the amount actually needed. The student will better understand this theory if he recalls his fundamental physiology. Living substance is characterized, among other things, by irritability which is instability. It is in a constant, state of unstable equilibrium. Whenever the equilibrium becomes permanently stable the substance is dead. It is also continually attempting to restore disturbances in its equilibrium. Whenever a chemical substance unites with a chemical substance in the cell, a receptor, the latter is, so far as the cell is concerned, _thrown out of function_ for that cell. The chemical equilibrium of the latter is upset. It attempts to restore this and does so by making a _new_ receptor to take the place of the one thrown out of function. If this process is continued, _i.e._, if the new receptor is similarly “used up” and others similarly formed are also, then the cell will prepare a supply of these and even an excess, according to Weigert’s theory. Whenever a cell accumulates an excess of products the normal result is that it excretes them from its own substance into the surrounding lymph, whence they reach the blood stream to be either carried to the true excretory organs, utilized by other cells or remain for a longer or shorter time in the blood. Hence the excess of receptors is _excreted from the cell that forms them_ and they become _free_ in the blood. These free receptors are termed _antibodies_. _They are receptors_ but instead of being retained in the cell are _free in solution in the blood_.