HORMONE FACTOES IN GROWTH AND DEVELOPMENT 203 



In protozoa, too, no effect in growth (rate of division) could be ob- 

 served if thymus was added to the culture-medium (Shumway). 



Thus the extirpation experiments as well as the feeding experiments 

 point at present to the conclusion that the thymus gland does not produce 

 any specific substances influencing growth or development. The feeding 

 experiments have shown that certain specific substances needed for normal 

 growth and development are absent from the thymus gland. 



It seems, therefore, justified to assume that the thymus gland is not 

 concerned in the control of growth and development. 



