165 



are found to l^e very strong in (issues which have a vilal 

 significance. 



The embr3'onic tissue juices contain substances wliicli 

 pj-omote growtli in vitro for an indefinite lengtli of lime "'^). 

 Extracts of tissues from the adult organism also ]K)sscss 

 growth-promoting substances but not of the same grade as 

 the embryonic tissue juices. Carrel 5^) has shown that 

 certain tissues give an extract with greater activating power 

 than others. Extracts of leucocytes or spleen possess nearly 

 as much activating substance as emljryonic tissue juice ^8). 

 The mechanism of growth and multiplication of tissue cells 

 is a profound and complex phenomenon. Besides the growth- 

 promoting factors found in the tissue juices, there may pos- 

 sibly be something, produced in the body of the living cell 

 or certain cells only, which initiates cell division and is 

 transported directly from one living cell to another. H a- 

 berlandt -'^i) 212) found that certain cells in the plant 

 tissue, the leptom-cells, liberate substances, which initiate 

 the cell division. Also Akamatsu') showed that substances 

 stimulating:, the cell division occurred in the blood, when 

 a wouna is made. 



The cytotropic character of tissue cells has already been 

 noted by several investigators. Roux*^^) was the first to 

 describe it. Recently R o u s ^s") has noticed the striking abil- 

 ity of isolated tissue cells to reunite. He never saw any 

 division or multiplication of isolated single cells. Also B u r- 

 r o w s ^1) mentions the important relation of cells in tissue 

 cultures to each other. He expresses himself as follows: •The 

 tissue culture cannot be compared in detail with the bac- 

 terial culture. The tissue cells planted in plasma do not 

 grow al the expense of the plasma. Single cells may show 



movement in this medium, but they do not grow 



Growth takes place only about fragments of (issue The 



nutrient material for the growth of these cells comes from 

 the cells disintegrating within the fragment." That the nu- 

 trient material for the growth of tissue cells comes from 



