STIMULATION OF GROWTH BY CROWDING 



155 



the sea urchin and starfish contain growth promoting substances. 

 These substances pass out into the surrounding water during seg- 

 mentation, and later stages of larval development There is 



55 



50 



45 



<t, 40 



35 



^ 30 



25 



20 



20 



30 



40 



Time 



50 



in Hours 



60 



Fig. 12. — Figure showing growth in length of plutei in the presence of animal 

 charcoal (A) and fuller's earth (x) compared with those growing in sea- water (O). 

 (From Peebles, 1929.) 



some experimental evidence in favor of the conclusion that the in- 

 hibiting substances are associated with the lipoid constituents and 

 the accelerating factor is contained in the protein molecule 



