89 



healthy looking cells. Therefore, only strains which have 

 been cultivated for a longer period of time in vitro (3 or 1 

 weeks'^ are good for experiments. Fresh tissue cannot be 

 conti'olled, the size (area and liiickness) cannot be measured 

 easily and besides this it contains usually all kinds of tissue 

 elements which comi)licale the experiments. On the con- 

 trary, tissue cultures with a big area of new growth, but 

 scattered around in a loose meshwork with rather big spaces 

 in between the cells, must be discarded for experiments. 



PREPARATION OF THE TISSUE.? 

 For experiments it is necessary always to have reliable 

 controls. Therefore, the selected culture has to be divided 

 into two or four j)arts and one of them serve as contix)! for 

 the others. It is necessary to make the control and experi- 

 mental culture out of the same fragment; there may be 

 differences in activity and structural conditions betw^een 

 tv^'o entirely different cultures. The tissue is extirpated fronn 

 the coagulum by four clean cuts with a sharj) cataract 

 knife. Any part of the coagulum which has nol been invaded 

 by the cells must not be left around the tissue. If the incisions 

 are not very clean, the coagulated plasma may become 

 twisted and folded and when placed in the new medium, 

 may prevent it from growing regularly. Hereafter the ex- 

 tirpated fragment is divided in as equal pieces as possible, 

 washed in Ringers solution and placed in the new media. 

 If the difference in size of the new fragments is too great; 

 this alone will cause a difference in rate of growth. It was 

 observed by Burrows that the new cells travel further 

 aw'ay from a big piece than from a small piece, probably due 

 to the accumulation of more waste products in the larger 

 piece than in the smaller. 



PREPARATION OF THE MEDIA. 



It is hardly necessary to mention that all chemicals used 

 lor preparing R i n g e r s solution have to be of the purest. 

 The glass wares used for preparing the media must be such 



