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Comparative Animal Physiology 



hormone. Testosterone injections are also known to be followed by some 

 nitrogen retention and weight gain. 



Some of the roles of hormones in mammals in influencing growth through 

 protein metabolism are diagrammed in Figure 277. 



It was discovered many years ago that the feeding of thyroid gland ma- 

 terial to amphibians hastens metamorphosis*^*^ and, conversely, that removal 

 of the rudiment of the thyroid from larval forms prevents metamorphosis.'^- ^^ 

 Metamorphosis can be obtained in thyroidectomized individuals by admin- 

 istration of thyroglobulin or thyroxin. Less effective is di-iodotyrosine, and 

 still less effective are inorganic salts of iodine or elemental iodine, which, 

 however, will induce metamorphosis if they are present in sufficient quanti- 



COj+H,0 



Fig. 277. Diagram summarizing the major influences of hormones on growth 

 in mammals. Modified from Long.^°^ 



ties.*^ This response to thyroxin is quite distinct from the increase in the 

 general rate of metabolism. Dinitrophenol, a powerful stimulant of metabol- 

 ism, has no influence on metamorphosis. Acetylated thyroxin, on the other 

 hand, retains its normal capacity to induce metamorphosis although it has 

 lost entirely its metabolism-stimulating action. 



Various amphibian species differ greatly among themselves in their re- 

 sponsiveness to thyroxin. During their early development, none of them shows 

 ability to respond to this hormone, but most acquire the capacity at some par- 

 ticular stage in their development. Some species, like the Mexican axolotl, 

 never develop any considerable degree of reactivity to this metamorphosis- 



