THE PITUITARY BODY 



are no new experimental data enabling us to decide whether 

 or not thyroid stimulation in amphibian larvae is due to a 

 principle different from that stimulating the mammalian 

 thyroid. 



SUMMARY 



The recent investigations of the various aspects of the 

 biology of the thyrotropic hormone have yielded a disap- 

 pointingly small crop of new facts. Much of the effort has 

 been expended in consolidating or extending slightly knowl- 

 edge which was already available. 



Probably there is general agreement on the principal func- 

 tions of the thyrotropic hormone. Its importance is great in 

 nearly all classes of vertebrates — i.e., to insure normal thy- 

 roid function, especially the discharge of thyroid secretion. 

 Unless thyroid hormone is actually liberated from the gland, 

 various striking effects occur in cold-blooded animals — i.e., 

 the metamorphosis of tadpoles cannot take place; in both 

 urodele and anuran amphibia molting or the desquamation 

 of cornified epidermis is absent; on the other hand, in reptiles 

 molting may either be accelerated or take place less fre- 

 quently. In both classes of animals, as in mammals, the rate 

 of heat-production is abnormally slow and cannot be adapted 

 readily to the demands of the environment. Atrophy of the 

 thyroid gland may or may not rapidly appear after hypophy- 

 sectomy. The invariable effect of the operation is to pro- 

 duce all the histological signs of inactivity of the thyroid, 

 such as the accumulation of densely staining colloid in vesi- 

 cles lined by flat epithelium, together with physiological evi- 

 dences of thyroid deficiency, such as a low rate of metabolism. 



Changes in the opposite direction are readily produced by 

 the injection of extracts of the anterior pituitary. The colloid 

 becomes vacuolated and may almost disappear. The epithe- 

 lium, now active, is cuboidal or columnar and begins to pro- 

 liferate. Animals of various classes exhibit changes de- 

 pendent upon thyroid secretion. Metamorphosis can be pro- 



[196] 



