25 



the cells themselves, but are far more ])rofoimd phenomena 

 correlated to the entire function of all the cells in the or- 

 ganism and their ''milieu inlerieure" (Carrel)"'^). 



The evident si<>nificance ol' this method was foreseen very 

 early by Carrel and a technique was developed by which 

 the rate of growth of the tissue in vitro c(ndd be measured. 

 Carrel and E b e 1 i n g ^'^"O worked out the ({uantitative 

 study of the growth of fibroblasts in vitro. In regard to the 

 imperfectness of the method and respecting the more or 

 less complicated phenomena dealt with, by measuring the 

 surface area of growth, the accuracy of the method was 

 found to be sufficient to discover new facts and the first 

 important step was made for studying i)hysiological phe- 

 nomena quantitatively. 



In order to develop a fairly safe technique one has to 

 have a more than ordinary interest for the problems and 

 a good deal of ])atience. The technique is not so difficult; 

 almost any person can be trained to do it. It is shown by 

 Ebeling^'^3) that if a culture is divided into two equal 

 parts and cultivated separately under uniform conditions, 

 the rate of growth does not vary more than 8—10 per cent, 

 which must be said to be very accurate in regard to the 

 im]:)erfectness and the complexity of the method. 



By employing the quantitative method very interesting 

 and important discoveries have already been made and by 

 this method unlimited problems of importance can now be 

 attacked. For example, the problems of senescence have been 

 investigated by Carrel : later C a r r e 1 and E b e 1 i n g 

 92 94 102) have shown that in plasma an inhibitory fac- 

 tor for the growth of connective tissue is contained, and 

 that cells grow^n in plasma of younger animals grow much 

 more abundantly than in that of older animals. The ^.ffect 

 of age is marked, the rate of grc^wth of fibi'oblasts in chiic- 

 ken plasma decreasing 50 per cent with the first three 

 years of life and 30 per cent more in the next six years 

 of the life of the fowls furnishing the ])lasma. And they 



