STIMULATION OF GROWTH BY CROWDING 



149 



The proportions of decrease and of growth or regeneration differ be- 

 tween isolated and grouped animals. The results for the 7 days the 

 animals were observed are given in Table VI. The results indicate, as 

 much as a single experiment is likely to, that there is a greater re- 

 generation with the decreased volume per animal, which is compen- 

 sated by the greater growth of the tail stump when the animals are 

 isolated. Bilski states that this experiment is supported by his gen- 

 eral experience in many similar experiments in other phases of the 

 work, but cites no direct support of these results. 



TABLE VI 



Showing the Effect of Crowding on the Rate of Regeneration of 



Tails of Frog Tadpoles in 7 Days. (In Millimeters) 



(Data from Bilski) 



B, length from tip of head to root of tail. 



5, length of tail stump after cutting. R, length of regenerated material. 



* Percentage B is calculated on the basis of the original body length; percentages 5 and R are in terms 

 of the original tail length before operation. 



CROWDING IN TISSUE CULTURES 



Work with tissue cultures has yielded pertinent evidence concern- 

 ing the beneficial effects of crowding on growth. The literature in 

 this field is enormous; and no attempt will be made to cover the 

 different ramifications of the subject, with most of which we are not 

 immediately concerned. It has been known for some years (Carrel, 

 1924) that tissues to be grown in vitro must have a proper back- 

 ground on which to creep. One of the most used backgrounds is of 

 fibrin network. In many recent studies this is placed as blood-plasma 

 in a thin layer over the bottom of a special culture flask. The tissue 

 to be cultured is introduced aseptically into this sterile medium, 

 which is then covered with a sterile fluid that has Tyrode solution as 

 its main ingredient but which contains other materials such as serum 

 or a saline extract of embryonic tissues. The latter, or some fraction 

 thereof, appears to be necessary for real growth of such cells as 



