98 THE SMALLEST LIVING THINGS 



type. In some strains of Paramecium an acid reaction persists 

 throughout, and in some forms, e.g., Actinosphaerium, active 

 digestion occurs only in an acid medium. 



In some cases, the ferments responsible for the digestion of 

 proteins have been isolated by extraction. In 1886 pepsin-like 

 ferments, which will dissolve albumin in an acid medium, were 

 obtained from one of the slime molds, Ethalium septicum, and 

 in 1893 from the ameba-like rhizopod, Pelomyxa, while trypsin- 

 like ferments, active in an alkaline medium, have been obtained 

 from a number of other forms. 



The method of determining the acid or alkaline character of 

 a gastric vacuole is rather interesting. Wilhelm Friedrich von 

 Gleichen, one of the early German physiologists in the eighteenth 

 century, discovered that minute granules of colored substances, 

 such as carmine or indigo which are practically insoluble in water, 

 will be taken into the gastric vacuoles of protozoa as readily as 

 will bacteria. The vacuoles thus become brilliantly colored and 

 remain so through the processes which result in the digestion of 

 ordinary food. 



Using this method and seeing one vacuole fill and move away 

 to be followed by another, and this by a third, etc., Ehrenberg, 

 in 1838, concluded that many of the protozoa differ from higher 

 animals in having numerous stomachs, and was led to the crea- 

 tion of a special group for them, the Polygastrica. The same 

 method of experimentation has been more fruitful in the hands 

 of modern observers, who, using substances which change color 

 in both acid and alkaline media, were able to tell the duration 

 of these chemical states. Thus in 1905 Serge Ivanoric Metal- 

 nikov, then director of the biological laboratory in the University 

 of St. Petersburg, Russia, used granules of alizarin sulphate, 

 which remain violet in color in the alkaline medium in which 

 Paramecium lives, but turn yellow after a short period in the 

 gastric vacuole. A still more refined method has been introduced 

 with the microdissection apparatus.* By the aid of this instru- 

 ment delicate chemical indicators in fluid form are injected into 



* A microdissection apparatus consists of extremely finely drawn glass tubes 

 which are attached to and manipulated by means of micrometer screws with vernier 

 readings. The whole assembly is placed on a microscope stand. In the hands of an 

 expert extremely delicate operations can be performed on individual cells. 



