i)2 



tin- rt'lalivc iiu-rcasc of Ihc conlrol fragment cxprossos the 

 quotiriil. {^rowlli intirx. 



I^ 1) (' I i n g " '■ showed, as niciilioiicd. Ihal if two frag- 

 ments derived from one eulture are eulli\aled under such 

 uniform conditions as (lescril)ed above, the (ntferences ob- 

 tained are less Hum 10 Oo. H requires, of course, a fairly 

 long period of practice and patience U) hi- al)le to trust one's 

 teehni(|ue. 



As it has lieen pointed out by E b c 1 i n g i*'-'), the value 

 of the growth of a |)iece of tissue is compared only with 

 the value of Ihe gi'owth of auolhei- piece of the same tis- 

 sue. Therefore, the absolute value of the growth is not of 

 great importance and factors which do not modify the rela- 

 tive value of the growth can be disregarded. The factors re- 

 ferred to are the different (jualities of the plasma, tissue 

 juices, variations in Hie lemjierature in the incubator, and 

 so on. 



Several investigators liave attempted to develoj) a tech- 

 nifpie by which it shoidd be possible to maintain tissues in 

 a condition of uninterrupted growth for long periods of 

 time, (Romeis ^^^\ B u r i- o w s -''). Fischer*)\ Tne tech- 

 nical difficulties were rather great, therefore, the principle 

 of a method of cultivating tissue cells in vitro has been 

 given up for a while. Quite recently Carrel^'') has taken 

 Uj) the i)r()blem and worked out a method, although far 

 from perfect, but indeed very important and useful. 



The problem consists of supplying the tissue cells with 

 the necessary food and at the same tinu' removing the 

 decomposition products from the culture without disturbing 

 the tissue and without bacterial contamination. 



Burrows-^ and Romeis^'^') have constructed com- 

 plicated apparatus which allow a continuous current of 

 solutions containing nourishing substances to overflow the 

 tissue. 1 *) have applied thin discs of elderberry pith as 

 support for the growing tissue while the pith was 

 ') Fisclier A., unpublistied experiments. 



