462 



ON INCREASE IN SIZE 



[PT. Ill 



in steps (see Fig. 55 a) as Brody would have it, or in a hyperbola 

 (see Fig. 55^) as Schmalhausen would have it. That Cohn & Murray 

 compared their results with the Minot curve does not matter, as 

 all percentage growth-rate curves, whether instantaneous or not, 

 fall towards the abscissa asymptotically. Cohn & Murray's achieve- 

 ment was to demonstrate that the growth-promoting impulse resides 

 to a large extent actually within the cells, and that its fading out 

 can be seen equally well in the 

 decreasing rapidity with which 

 isolated cells will grow in culture 

 as in the decreasing rapidity with 

 which the whole cell-population 

 of the embryo increases in the 

 egg. In other words, functional 

 changes taking place in the or- 

 ganism as a whole are mirrored x n 

 by similar changes in the in- 

 dividual cells. 



Cohn & Murray also deter- 

 mined the latent periods in their 

 chick heart cultures, and were 

 able to show that, as the embryo 

 grew older, so the cells of its 

 heart took longer and longer to 

 accustom themselves to their in 

 vitro environment. The latent 



Days 4 5 6 7 



8 9 10 11 12 13 U 15 16 17 18 

 Incabation age 



Fig. 64. 



Heart-fragments in plasma, ; 

 heart-fragments in plasma + Ringer, H- 



period was defined as the time intervening between the incubation 

 of the cultures and the first appearance of cells protruding from the 

 peripheral margin. As Fig. 64 shows, this value markedly increases, 

 though not until after the 7th day. Unknown to them, Suzuki had 

 previously found just the same relationship between latent period and 

 age in the chick embryo. His figure (Fig. 65) is a beautiful illustration 

 of their conclusions. Growth-rate and latent period are two strangely 

 associated processes. "When older tissue", as Cohn & Murray put it, 

 "i.e. a heart fragment from a 16-day old chick embryo, is placed in a 

 fresh environment, it does not assume immediately a divisional velocity 

 typical of younger tissue. In our experiments the previous growth- 

 rate was not approached for 36 hours. In other words, a number of 

 cell-divisions (i.e. generations) were required for it to lose the habit 



