156 Regeneration 



animal). No other material seems to have such an 

 effect. The thyroid contains iodine, and Morse 1 states 

 that if instead of the gland, iodized amino acids are 

 fed to the tadpole the same result can be produced. 

 We must, therefore, draw the conclusion that the normal 

 outgrowth of legs in a tadpole is due to the presence 

 in the body of substances similar to the thyroid in their 

 action (it may possibly be thyroid substance) which 

 are either formed in the body or taken up in the food. 



Thus we see that the mesenchyme cells giving rise to 

 legs may lie dormant for months or a year but will grow 

 out when a certain type of substances, e. g., thyroid, 

 circulates in the blood. There may exist an analogy 

 between the activating effect of the thyroid substance 

 and the activating effect of the spermatozoon or butyric 

 acid (or other parthenogenetic agencies) upon the egg, 

 but we cannot state that the thyroid substance activates 

 the mesenchyme cells by altering their cortical layer. 



The fact that the substance of the thyroid may 

 induce general growth in the human is too well known 

 to require more than an allusion in this connection. 

 When growth stops in children as a consequence of a 

 degeneration of the thyroid, feeding of the patient with 

 thyroid again induces growth. It may also suffice 

 merely to call attention to the connection between 

 acromegaly and the hypophysis. 



It was formerly believed that the nervous system 



' Morse, M., Jour. Biol. Chem., 1914, xix., 421. 



