23 



laining only inorganic salts (Ringers solution or sca- 

 watcrj and the surface of the cover slip can be used 

 as the only supporting a])paratus for the wandering of 

 the cells. This method cannot actually be called tissue 

 cullivalion. It is survival only of fragments of tissue in 

 vitro, a survival in a j) r o t e c t i v e solution which 

 is not a g r o w I h - p r o m o t i n g, nutrient solution, 

 The tissue cells live and multiply at the expense of the 

 nourishing and growth-promoting substances contained in 

 the explanted piece of tissue. 



It is perfectly true, as Burrows has stated, that fresh 

 embryonic tissue grows not because of the medium, 

 but in spite of the m e d i u m. The disintegration of 

 some cells liberates growth-promoting substances for other 

 cells, naturally within certain limits. The only true cul- 

 tivatioi of tissue is the method which deals with perma- 

 nent strains of various tissues. Then we can speak of an 

 actual growth in vitro, the advantages of this method being 

 namely, that the conditions of the experiments are much 

 simpler and more constant. The permanent strain is pure 

 and under uniform conditions all the time and not in- 

 fluenced by the many factors of the organism which are 

 still there immediately after explantation. To learn anything 

 about the influences of the various components of the 

 culture medium, or whenever certain problems have to be 

 investigated, it is necessary to carry on the strains over 

 a certain protracted period of time to be sure that the ex- 

 planted tissue has been adapted to the ncAv experimental 

 environment. The above-mentioned experiments of Lewis 

 and Lewis, 327) for instance, demonstrate the fact that 

 fresh explanted tissue is able to live in simple salt solutions 

 and grow for a very short time at the expense of the 

 substances in the tissue itself, but when it is attempted 

 to transfer the tissue from one salt solution to another it 

 usually results in the death of the tissue. Consequently it 

 is rathei' confusing when investigators, who have not w^orked 



