SALTS ARE NECESSARY 37 



so that the result is that the 1-per-cent jelly may 

 contain quite a variety of substances, and if some 

 human cells are placed on its surface we may try 

 the effect on those cells of any of those materials 

 which have been added in solution to the 2-per-cent 

 agar. We are thus able to investigate, by a method 

 which is simplicity itself, the effects of drugs or 

 chemical' substances upon the individual human cell. 



Before we begin to discuss this subject, however, 

 we must be certain that the cells are alive when they 

 are being subjected to the drug. It is, of course, 

 well known that when, say, a drop of blood is re- 

 moved from the finger the leucocytes are alive; but 

 it is necessary to be certain that they are not killed 

 immediately they are placed on the jelly-film. As 

 will be discussed at greater length later on, we can 

 always ascertain whether white blood-cells are alive 

 or not by mixing a certain quantity of an alkaloid 

 with the jelly; for alkaloids excite amoeboid move- 

 ments, and it is obvious that these movements cannot 

 occur in a dead cell. Since alkaloids have supplied 

 the means of determining this point, we have also 

 been able to ascertain how to make the jelly so that 

 it will keep the cells alive as long as possible; for it 

 is clear that a jelly which will allow cells to remain 

 excited for the longest period with a given quantity of 

 alkaloid must be the best jelly for keeping the cells 

 alive when made without the alkaloid. The presence 

 of a combination of certain salts is essential. 



Suppose a drop of blood is placed on to a film 

 of jelly which contains only agar and water and no 



