﻿STUDIES IN PLAGUE IMMUNITY. 173 



Semnopithecits cntellas Cuvier in relation to susceptibility to plague 

 infection. However, since all three of these species apparently belong 

 to different genera, it is perhaps not strange that their susceptibility to 

 the infection should vary to a certain extent. 



Obviously, in the beginning of this work it was important to determine 

 the lethal dose with a pest culture of known virulence for the species 

 Cynomolgus pMlippinensis with which the experiments were to be per- 

 formed and this was done as accurately as possible. However, the 

 susceptibility of the individual monkeys of this same species was found 

 to vary considerably. Usually, the animal dies from pest infection in 

 from three to seven days if the growth from a 48-hour agar slant culture 

 of the strain "Pest Virulent" is suspended in 5 cubic centimeters of 

 bouillon, and a 5 or 10 cubic centimeter syringe needle dipped in this 

 suspension and then thrust beneath the skin near the root of the tail. 

 Also, if the skin is shaved over a small area and slightly scarified and 

 then a suspension in bouillon of the same culture is rubbed over this 

 area, the animal usually succumbs. On the other hand, some of these 

 monkeys will survive the inoculation of similar and of even much larger 

 amounts of the virulent pest organism, certain of them remaining alive 

 after the injection of even \ and -^ oese of "Pest Virulent". However, 

 it has been found that -J oese of "Pest Virulent" suspended in 0.25 

 cubic centimeter of bouillon always constitutes a fatal dose for mon- 

 keys averaging about 2,000 to 3,000 grams in weight, and therefore, 

 although usually this quantity of plague culture really represents many 

 times the multiple lethal dose for the majority of these animals, it has 

 been employed in testing the immunity of all the monkeys used in my 

 experiments and weighing under 3,000 grams, excepting in the first 

 series of experiments where the lethal dose had not been determined 

 accurately and where it is then noted in the tables. For monkeys over 

 this weight § oese of "Pest Virulent" suspended in 0.33 cubic centimeter 

 of bouillon has been employed. In order that the size of dose used 

 in testing all the animals might be as uniform as possible one 48-hour 

 agar slant culture of the strain "Pest Virulent" was suspended in 5 cubic 

 centimeters of bouillon or saline solution; 0.25 cubic centimeter of the 

 suspension then contains i_ oese and 0.33 cubic centimeter § oese of this 

 organism. 



The variations in susceptibility between different individuals of this 

 species of monkey are evidently of considerable importance in the study 

 of their immunization and, as will be seen from the experiments which 

 will be related further on, this individual variation probably accounts 

 largely for the different results in immunization which have been obtained 

 in animals of the same weight and inoculated with the same dose and by 

 the same method. 



If, for example, each animal of a series is inoculated with a fixed 

 amount of plague vaccine and, later, the immunity of each is tested by 



