CHAPTER LIV 

 CONDITIONS FOR DEVELOPMENT 



332. Development and life. The conditions for the develop- 

 ment of young plants are the same as the conditions for growth 

 or for being alive. Development is, in fact, one of the aspects 

 of being alive — just as assimilation, energesis, irritability, etc. 



^t 



t 



Fig. 121. Influence of thyroid on development 



In each series, b shows the tadpoles that had been fed on thyroid much more advanced 



in development, although smaller, than the controls, a. The control animal in / was fed 



on plant food, in 2 on muscle, in 3 on adrenal. ,"\.ll full size. (.A.fter Gudernatsch) 



are aspects of being alive. But development goes on so 

 slowly in most of the familiar plants and animals that most 

 people do not notice it as a distinct fact until their attention 

 is called to it. Changes in the external conditions influence 

 the development of animals, just as they influence the devel- 

 opment of plants. 



That the development is not altogether a matter of growth 

 has been shown over and over again. In certain experiments 

 conducted by Dr. J. F. Gudernatsch, of the Cornell Medical 

 College, tadpoles were fed on thymus glands obtained from 



285 



