66 DIFFUSION OF SUBSTANCES INTO LIVING CELLS 



obtained by in-vitro experimentation have been con- 

 firmed in some instances by experimentation in the 

 living body, and hence one may, I think, reasonably 

 infer what goes on in vivo from what is observed 

 in vitro, and that these experiments into individual 

 cells may be undertaken with confidence. 



Before continuing this subject another matter con- 

 nected with it must also be stated. In the previous 

 chapter it was mentioned that cells observed in vitro 

 must be resting in a solution or on a jelly which con- 

 tains certain salts the presence of which are necessary 

 for keeping them alive. In the body one of these salts, 

 sodium chloride, is actually present; but there is no 

 sodium citrate, a solution of which has proved to be 

 the best one for leucocytes and other cells to live in. 

 Obviously in the body there must be some salt or salts 

 for which the sodium citrate is a substitute or equiva- 

 lent. One of the roles played by sodium citrate in 

 in-vitro experimentation is its property of preventing 

 coagulation of the blood, which seems to be an im- 

 portant one, for related to this is the curious fact that 

 leucocytes will live longer in citrated plasma than in 

 undiluted serum, a point which will be alluded to in 

 the description of the method of measuring the lives of 

 leucocytes. Sodium citrate, however, is detrimental to 

 leucocytes, and there is no solution known which will 

 keep leucocytes or other human cells alive for more 

 than a few days. If there was we should now be in 

 a position to cultivate families of human blood-cells in 

 test-tubes. At present, by means of sodium citrate, 

 one can only make leucocytes "exist" for some hours 



