70 PROTOPLASM 



before knowledge of this had progressed much beyond the pure 

 salt-solution stage, other important problems were partially 

 solved, notably that of a suitable substratum for the cells to 

 grow upon. Cells must have a solid surface to cling to if active 

 growth is to take place. The best substratum so far used is 

 blood plasma (plasma is the medium in which the corpuscles of 

 blood are suspended). Agar, gelatin, glass wool, silk gauze, 

 spider webs, cotton fibers, etc., have all been tried, but while 

 any one of them can be used as a framework upon which the cells 

 may grow, none gives so uniform a growth as does the fibrin in 

 plasma. Lymph was first used (lymph is a body fluid and coagu- 

 lates like blood), but later Burrows substituted the plasma of 

 blood. It is obtained by centrifuging whole blood; the upper 

 layer is plasma. The liquid plasma is put in a Carrel flask. 

 Coagulation quickly follows. On this coagulum the tissue is 

 placed and covered with nutrient solution. 



Still another condition necessary to keep tissues alive for a 

 great length of time is washing. The flask must be opened, or 

 the slides unsealed, the tissues thoroughly washed (with the 

 nutrient or a saline solution), a fresh supply of the culture medium 

 added, and the respective receptacles resealed. Carrel describes 

 his discovery of the need of washing as follows : At the beginning, 

 the cultivation of tissues consisted in the brief survival of a frag- 

 ment of fresh tissue in a drop of body lymph or blood plasma ; but 

 after a few days, the cell degenerated and died, owing to waste 

 products, lack of food, or both. Washing in Ringer solution 

 every two or three days proved remarkably successful in pro- 

 longing the duration of life. Tissues which are to grow for 

 months or years must be regularly and frequently washed, yet 

 when neglected they may show surprising vitality. A bit of 

 chick heart, sealed in a hollow-ground slide, immersed in culture 

 medium and left untouched for 18 days may still exhibit active 

 beating. Washing, however, prolongs the time. Carrel tells 

 of a fragment of heart which continued to pulsate 104 days after 

 its extirpation from a chick embryo. Cells migrated, multiplied, 

 and remained normal. But the total mass of the tissue did not 

 increase. A suitable substratum and frequent washing are not 

 sufficient. Nutrition, other than a pure salt solution, is necessary. 



The question of a suitable nutrient medium for the cultivation 

 of animal tissue and of plants has a long history, beginning some 



