SECT. 2] 



AND WEIGHT 



463 



100(— 



of slow growth previously imposed upon it by its organised environ- 

 ment. This happening is an expression of a process of rejuvenescence 

 or dedifferentiation*. The tissue soon assumes a growth-rate deter- 

 mined by its environment, and as long as the environment can be 

 kept relatively stable, that is to say of uniform composition, pre- 

 sumably there will be no further change in divisional velocity." 



As in the intact embryo, then, negative acceleration of growth 

 is greatest at the beginning of 

 life. On the other hand, the 

 latent period before growth in 

 vitro begins is greatest at the end 

 of embryonic life. It is likely, 

 therefore, that the factors which 

 determine the initiation of 

 growth and those which deter- 

 mine the extent to which it shall 

 take place in a given time are 

 not identical. 



Olivo & Slavich and Oda & 

 Kamon, subsequently repeated 

 the experiments of Cohn & 

 Murray and as far as heart 

 fragments were concerned, con- 

 firmed them in every particular. 

 Nordmann also confirmed them 

 for liver cells. Oda & Kamon 

 found that the growth-rate in 

 culture was the more rapid the younger the embryo from which 

 the heart had been taken, and the latent period was the shorter. 

 The pulsatile activity of the heart explants was also more hardy the 

 younger the embryo. On the other hand, they did not obtain such 

 clear-cut results with the spleen which to some extent showed a 

 higher in vitro growth-rate the older the embryo from which it had 

 been taken. This doubtful point urgently requires re-investigation, for 

 it has become a commonplace of explantation research that growth- 

 rate in vitro follows growth-rate in vivo. Moreover, Allen has found 

 other differences between organs ; thus in saline, embryo heart will not 

 grow at all after the 8th day, intestine after the i ith day, and so on. 



* This does not imply the acquisition of any pluripotence or totipotence. 



5 10 15 



Age of chrck giving the heart-cells 



Fig. 65. 



