l.ll 



111 piTcipitaling llu' serum willi CO,, it was loimd iluil 

 these substances, globulins, increased the aelivily ot til)i-(»- 

 blasls, which indicates that a growlh-pronioting substance 

 had been obtained from the serum. The serum deprived 

 of its globulins showed to have an increased iiihibiling effect 

 on the fibroblasts. It was therefore concluded that the re- 

 straining effect of serum on the activity of homologous fibro- 

 blasts is due in i)art to the antagonistic action of growth- 

 activating and inhibiting substances. 



The growth-promoting principles of the serum, which 

 is precijjilaled by CO. is found by Carrel and Ebe- 

 ling35) to be as unstable as alexin and cerlain tissue 

 juices, which have the property of increasing the rate of 

 cell proliferation. The alexin and the growth-promoting sub- 

 stances contained in embryonic tissue juice possess the 

 same property of being destroyed by heat and by shaking. 

 It was found that the inhil)iling action of shaken homolo- 

 gous serum on fibroblasts was at least 30 per cent greater 

 than that of unshaken serum. 



When heterologous serum was shaken, dog serum being 

 used, its inhibiting action on chicken fibroblasts was very 

 much decreased. The hemolytic; action of dog serum to- 

 wards the red blood corpuscles of chicken, was partly in- 

 activated by shaking. The shaking could be com])ared to 

 the influence of heat. Shaken serum was found always to 

 inhibite the activity of homologous fibroblasts much more 

 than normal unshaken serum. When the alexin of chicken 

 serum was completely inactivated by shaking, its restraining 

 power on chicken fibrol)lasts became also more marked. 

 On the contrary, dog serum partly inactivated by shaking, 

 was much less toxic for chicken fibroblasts than normal 

 serum It was assumed by Carrel and Ebeling^^; tliat 

 the restraining effect of shaken serum on foreign fibroblasts 

 may be attributed to the destruction of alexin. 



After Carrel f''0 had introduced his new technique of 

 cultivating tissues in flasks uninterrupted for 2—3 weeks 



