144 



VETERINARY BACTERIOLOGY 



atmosphere, but more easily by covering the medium by a layer 

 of paraffin or other neutral oil. The methods of preparing the 

 toxin for use and of manufacture of the antitoxin do not differ 

 materially from those used in the production of diphtheria anti- 

 toxin. 



Unlike diphtheria toxin, the tetanus toxin may be dried and 

 preserved indefinitely without deterioration. It is, therefore, the 

 toxin and not the antitoxin which is sent out from the Hygienic 



Fig. 72. A filtration apparatus after Uhlenhuth and Weidanz. 



Laboratory to the serum laboratories for the purpose of standard- 

 ization. A standard toxin has been prepared at this laboratory, 

 and its M. L. D. for a 350-gm. guinea-pig carefully determined. 

 This is sent out in dried form, and is diluted before use, so that each 

 cubic centimeter contains 100 M. L. D. of the toxin for a 350-gm. 

 guinea-pig. The immunity unit is defined as follows: " The 

 immunity unit for measuring the strength of tetanus antitoxin 



