24 



imicb in this line lu't'oi-c, shilc lli;il. lor example, lissiic 

 can he cnllivalcMl in so simple :i nudiiim as ■sca-walcr . 



II is impoi'lani llial some dislinclioii he made hclwciMV 

 the two melhods ol' invesligalion. the one which deals with 

 experimenis on s n r \ i v i n <^ lissne in pi'()lecli\o 

 solutions and Ihc olher one, (he in \' i t r o c u i I i- 

 V a I i o n of lis s u e in cull u r e m e d i a w h i c h a c- 

 lually |)|-om()le the ^ r o w I li. 



This lacl seemed to h;' wry lillle understood hy several 

 invesli<.>alors; only \-ery few peoble work with tissue enlli- 

 valion in a confident manner, whatever may he the cause: 

 lack of leclinical difficnllies oi- underslandinff. 



III studying physiological and pathological prohlems it 

 is iherelore of great importance to carry on the experimenis 

 over a long period of lime and to nse permanent strains of 

 tissues which have acquired a certain grade of stability 

 for the in vilro life. It was taken for gi'anled at the 

 early ])eriod, when the techni(iue for tissue cultivation came 

 up, thai the blood plasma contained all the substances for 

 the nourishment of the tissue in the culture. Of course 

 there are some; but cx|)eriments have recently showm that 

 the plasma or serum has a more inhibitory than a 

 growth promoting influence (C arret and E b c 1 i n g) i"') 

 and this fact could only be abstracted from a nu'thod 

 which has the jiurpose of following the rate of growth 

 over a long period of time. And the same !nethod 

 resulted also in finding those substances which are 

 absolutely required to make the tissue live indefinitely. 

 It was early found by (] a r r e 1 '**') ^8) that the juice from 

 young embryos contain all the necessary sul)stances for the 

 indefinite life and that tissue juice and blood plasma from 

 adult and old individuals do not contain these substances in 

 any amount sufficient for the indefinite life. In this connec- 

 tion it may be mentioned that the tissue cells from young 

 and old animals can be cultivated indefinitely, i. e., Ihal it 

 seems as if the factors of senescence are not to be foimd in 



