able _a priori_, and if established removes the
necessity for the third. It is strongly supported by Ehrlich, who, in his
so-called "side-chain" (_Seitenkette_) theory, explains antitoxin
production as an instance of regeneration after loss. Living protoplasm, or
in other words a biogen molecule, is regarded as consisting of a central
atom group (_Leistungskern_), related to which are numerous secondary atom
groups or side-chains, with unsatisfied chemical affinities. [Sidenote:
"Side-chain" theory.] The side-chains constitute the means by which other
molecules are added to the living molecule, _e.g._ in the process of
nutrition. It is by means of such side-chains that toxin molecules are
attached to the protoplasm, so that the living molecules are brought under
the action of the toxophorous groups of the toxins. In antitoxin production
this combination takes place, though not in sufficient amount to produce
serious toxic symptoms. It is further supposed that the combination being
of somewhat firm character, the side-chains thus combined are lost for the
purposes of the cell and are therefore thrown off. By the introduction of
fresh toxin the process is repeated and the regeneration of side-chains is
increased. Ultimately the regeneration becomes an over-regeneration and
free side-chains produced in excess are set free and appear in the blood as
antitoxin molecules. In other words the substances, which when forming part
of the cells fix the toxin to the cells, constitute antitoxin molecules
when free in the serum. This theory, though not yet established, certainly
affords the most satisfactory explanation at present available. In support
of it there is the remarkable fact, discovered by A. Wassermann and Takaki
in the case of tetanus, that there do exist in the nervous system molecules
with combining affinity for the tetanus toxin. If, for example, the brain
and spinal cord removed from an animal be bruised and brought into contact
with tetanus toxin, a certain amount of the toxicity disappears, as shown
by injecting the mixture into another animal. Further, these molecules in
the nervous system present the same susceptibility to heat and other
physical agencies as does tetanus antitoxin. There is therefore strong
evidence that antitoxin molecules do exist as part of the living substance
of nerve cells. It has, moreover, been found that the serum of various
animals has a certain amount of antitoxic action, and thus the basi