STIMULATION OF GROWTH BY CROWDING 153 



are known to be harmful to cells whether grown in vitro or in vivo. 

 Archusia have properties resembling bios and vitamin B and have 

 been thought to be identical with the latter. 



The whole concept of archusia is in the hypothetical stage at pres- 

 ent, and more evidence is needed before coming to a definite con- 

 clusion concerning its validity. 



Heaton (1926) has worked upon the effect of vitamin B upon the 

 growth of cells in vitro. He finds two elements present in extracts of 

 yeast and of liver — one which stimulates growth and another which 

 depresses it. The two can be separated by their different solubility in 

 alcohol. Burrows and Jorstad (1926) think that vitamin A is neces- 

 sary for the functioning of cells and is produced when cells are digest- 

 ing fats and growing under the stimulus of relatively high concentra- 

 tions of archusia (vitamin B?). They regard vitamins A and B as 

 antagonistic, and balanced in cells that are functioning. In fact, 

 most observers are agreed that fats and lipoids are associated with 

 substances which inhibit the growth of cells, while some portion of 

 the protein molecule is associated with the promotion of growth. 



EFFECT OF CROWDING ON GROWTH OF SEA-URCHIN PLUTEI 



Certain of these results of the tissue culturists have been applied 

 to the problem of the effect of crowding upon the rate of cleavage and 

 of the growth of the arms of sea-urchin plutei by Peebles (1929). By 

 treating extracts of sea-urchin eggs and larvae with alcohol or with 

 acetone, a growth-inhibiting substance was obtained which definitely 

 retarded the rate of growth of eggs or of plutei. When this fraction 

 containing lipoids was partially removed, growth acceleration was 

 observed, as shown in Figure 10. Further experiments showed that 

 there was a decided difference in growth, depending on whether the 

 alcohol-soluble or alcohol-insoluble fractions of extracts of echino- 

 derm plutei were used. These results are shown graphically in Fig. 1 1 . 

 Peebles was also able to remove growth-inhibiting substances from 

 such extracts by adsorption, as shown in Fig. 12, but has not been 

 able as yet to isolate either the growth-inhibiting or the growth- 

 promoting principle. 



Peebles, in summarizing her work, says: "The eggs and larvae of 



