250 THE DIVISION OF LEUCOCYTES 



for twenty minutes while mitosis has been induced, but 

 this experiment has only been followed by success on 

 very few occasions. For general practical purposes it 

 must be remembered that whatever is done in the way 

 of attempting to induce divisions in cells when they are 

 resting on a jelly-film, this must be done in ten minutes. 

 If the diffusion of the chemical agent is delayed beyond 

 this time, except in very few instances, the cells will 

 refuse to divide at all, simply because they die. 



The reason for this is a question of vitality, which 

 brings us back to the disadvantages of in-vitro experi- 

 mentation. All cells of the body lose vitality gradually 

 after they have been shed. White blood-corpuscles will 

 live in citrate solution for two or three days at the room 

 temperature, but they lose vitality all the time. As 

 already pointed out, there is no known medium in 

 which blood-cells will live and thrive, and in the best 

 medium at our disposal they merely exist for this short 

 period. When cells are resting on a jelly-film, how- 

 ever, they are not even in the best available medium; 

 but at present we are bound to employ the jelly 

 method, for we have not succeeded in inducing divisions 

 in any other way. The reason for this is two-fold: 

 firstly, because substances can be made to diffuse into 

 individual cells more quickly if the cells are pressed 

 into the jelly which contains them; and, secondly, we 

 think that lymphocytes prefer to be at rest when they 

 divide, for we cannot induce divisions \vith the cells 

 floating in a solution, although we have tried to do 

 so many times in solutions which have contained the 

 necessary constituents. 



