STUDIES ON THE SERUM OF VACCINATED ANIMALS 33 



well shown in their change in shape and reaction to dyes within the 

 body of the phagocyte. We shall consider in the following section 

 the nature of these changes within the phagocyte. There is a 

 simple technical method that has permitted us to study with 

 accuracy and facility a good number of bacteria from this stand- 

 point. 



III. Phagocytosis in Vitro. The Bactericidal Property 



of Phagocytes. 



Phagocytosis may take place not only in the animal body, but 

 also in vitro. The phenomenon of phagocytosis occurs rapidly in a 

 mixture of an exudate rich in phagocytes and a bacterial emulsion, 

 if it be placed in a moist chamber at a temperature of 35 degrees. 



A good way to obtain an exudate containing a large number of 

 leucocytes is to inject a few cubic centimeters of peptone bouillon 

 into the peritoneal cavity of an animal. For example, if a guinea- 

 pig is given 3 c.c. of bouillon into the peritoneal cavity, the exudate 

 on the following day is found to contain large numbers of white 

 corpuscles, as Issaeff * pointed out. In an exudate obtained in this 

 manner there are found large numbers of polynuclear amphophiles, 

 considerably fewer mononuclear leucocytes and a very few true 

 eosinophils (that is to say, leucocytes with large round eosinophilic 

 granules); these latter are in such small numbers that they are 

 frequently not to be found on a single slide. The exudate may be 

 drawn from the animal by puncturing the belly wall with the point 

 of a finely drawn out pipette. As soon as the peritoneal cavity is 

 reached the exudate rises in the tube to considerable height. 



A drop of this exudate is placed on a cover slip. A platinum 

 loop full of the culture fluid, in which the organism to be studied 

 has grown, is then added to this small drop and mixed with it. 

 The cover slip is then placed on a hollow ground slide and sealed 

 with vaseline. After incubation for a while, stained preparations 

 are made from this hanging drop. Preparations stained with eosin 

 and then methylene blue (Ehrlich) are very satisfactory. The ex- 

 udate is spread out on the slide and, after drying, is fixed either by 

 heat (Ehrlich's method) or, better still, by a 5 per cent solution of 

 carbolic acid or a saturated solution of picric acid. The latter 



* Issaeff, Zeitschrift fur Hygiene, t. XVI, 1894. 



