66 STUDIES IN IMMUNITY. 



And this idea proved to be correct. We then tried recently 

 obtained serum from the same animal. This fresh serum brings 

 about transformation of the vibrio without the addition of normal serum. 

 The phenomenon, to be sure, was not complete; there were a few 

 vibrios that remained unaltered and others that seem to have been 

 killed without any modification, since they failed to take the stain. 

 The immunized goat serum is not as suitable a medium in which to 

 demonstrate transformation as is the serum or blood of a vaccinated 

 guinea-pig. Fresh immune serum from this latter animal also brings 

 about complete transformation without the slightest necessity for the 

 addition of normal serum. 



Let us consider, first, a preventive serum which does not alone 

 cause the granular transformation. Since the aid of normal serum 

 is necessary, we may better study the influences that affect its 

 activity. 



All the following experiments have been modeled after a funda- 

 mental experiment and, in order to be comparable with it, they 

 have all been performed in an identical manner; a single factor, the 

 normal serum, has been modified. The hanging drops examined 

 were always made in the same way: 



The hanging drops were made from two equal-sized drops placed 

 side by side on the slide and then mixed. These two drops were 

 each from the same platinum loop. One of the drops consists of 

 the fluid whose bactericidal property is tested (in this case the nor- 

 mal serum) and the other consists of an emulsion of vibrios (24-hour 

 agar culture suspended in 6 c.c. of normal salt solution) mixed or 

 not, as the case may be, with the preventive serum. This latter 

 mixture was made as follows: A drop of the emulsion of vibrios 

 was dropped from a capillary tube into a watch glass and to it a 

 large platinum loop of preventive serum was added. The pre- 

 ventive serum, then, in the final hanging drop is very small in 

 amount, much less than the normal serum. Controls were also 

 made in each instance by using an emulsion to which no preventive 

 serum had been added. An experiment may be represented by 

 this abbreviation: 



Cholera + preventive goat serum 4- normal serum, 

 or 



Cholera + normal serum. 



