468 



ON INCREASE IN SIZE 



[PT. Ill 



After these demonstrations of the nature of the growth-promoting 

 factor, the assertions of other workers that a hormone is in reaHty 

 concerned do not seem very convincing. The paper of Heaton may 

 be consuked for experiments done from this point of view. The fact 

 that tissue cuhures of very early chick embryos are not successful 

 and the fact that observers are agreed on the necessity of keeping 

 the cells in numbers for successful cultivation led Wright to investigate 

 how it is that in nature the early stages of the chick are ever success- 

 fully passed through. Yolk from the hen's egg proves itself inactive 

 when added to tissue cultures, probably because, as Carrel & Baker 

 showed, yolk-lipoids inhibit growth in explants, but Wright found 

 that on dialysing the yolk a sub- 

 stance passed through the membrane ^ 

 which behaved very similarly to the i; 

 dialysates from embryo tissue juice. 5 

 Thus in one experiment where the -^ 

 yolk diffusate from 7-8-day eggs was — 

 tested on heart fragments of from E 

 10- 1 1 days' incubation, the number {£ 

 of mitoses in the treated explant was "S 

 from 1 14 to 127, while in the control «^ 

 it was only 14. Willmer has discussed 



160 



140 



120 



100 



80 



60 



these observations in an interesting ° 



o 



40 



20 



Age 

 Fig. 66 



6 

 n Years 



10 



review. 



Fischer has elaborated a theory of J^ 

 "desmones" or mutually stimulat- q 

 ing substances, and Burrows has 

 introduced the terms "archusia" 

 and "ergusia" for similar ideas. It 

 is not possible to review the work of these authors here. Burrows & 

 Jorstad did, however, point out that the growth of embryonic cells in 

 tissue culture depends on sufficient crowding and a certain stagnation 

 of the medium. For reasons not at all obvious to the bio- 

 chemist. Burrows & Jorstad identified archusia with vitamine B 

 and ergusia with vitamine A. Still less satisfactory is the work of 

 Carnot, of Roulet and of Carnot & Terriss who affirm, on very 

 slender evidence, that wounds of metazoal animals heal much quicker 

 when treated with embryo extract than when untreated. Carnot & 

 Carnot have also maintained that injection of foetal extracts into 



