CYTOTOXINS. 63 



ing them five or six injections of 10 to 20 grammes 

 of dog brain and spinal cord mixed with normal 

 salt solution. The serum of these ducks injected 

 intracerebrally into dogs in doses of one-half c.c. 

 caused the dogs to die almost at once in complete 

 paralysis, whereas if normal duck serum was in- 

 jected in the same way no effects of any kind were 

 produced. If smaller doses of the specific neuro- 

 toxic serum were administered, say o.i to 0.2 c.c., 

 various paralyses and epileptiform convulsions set 

 in, from which the animals sometimes recovered. 

 The action of this serum is also specific, i.e., the 

 serum of ducks treated with dog brain causes 

 these symptoms only in dogs, while on rabbits it 

 acts no differently than normal duck serum. 



These are the most important of the cytolytic 

 or cytotoxic sera, though, of course, the list can 

 readily be extended experimentally. In all this 

 we are evidently dealing with a general biological 

 law which we can express somewhat as follows: 

 An animal, species A, into whose body are injected 

 cells or cell products of species B, reacts by producing 

 specific substances in its serum against these cells or 

 cell products, provided, of course, that the incorpo- 

 rated cells or cell products find fitting receptors in the 

 body of A. 



Practical Applications of the Cytotoxins. With a 

 number of these sera therapeutic experiments 

 have been made in the human subject Those of 

 Metchnikoff and his pupil Besredka deserve men- 



