DUCTLESS GLANDS AND 

 DEVELOPMENT -II* 



Amphibian Metamorphosis Considered as Consecutive Dimorphism, 

 Controlled by the Glands of Internal Secretion 



Julian Huxley, 

 Oxford University, England 



"CRETIN" AND NORMAL TADPOLES 



Figure 1. On the left are two thyroidless tadpoles, showing extremes in size. On the 

 right are two frogs from the same "brood," and of the same age as the tadpoles. The 

 frog on the extreme right had his thyroid removed, but examination showed that part of 

 the gland had been left, and this fragment was sufficient to produce metamorphosis. The 

 distortion of the large tadpole's tail is an abnormality from which both normal and thyroid- 

 less tadpoles suffered, supposed to have been due to some impurity in the water. Lack of 

 the thyroid gland does not have an adverse effect on the growth of tadpole tissues. In fact, 

 "tadpole constitution" develops to an unusual size, but typically adult tissues are greatly 

 retarded or do not develop at all. Photograph reproduced by permission of Dr. Bennett M. 

 Allen, and the Journal of Expcri)ucntal Zoology. See text, p. 11. 



IF WE try to penetrate a little deep- internal environment really differ in 



er, we shall be driven to the con- regard to metabolic rate, the basal 



elusion that, since the thyroid, so metabolism of the larval stage being less 



far as is known, has always as its than that obtaining at metamorphosis, 



main function the acceleration of We then conclude that the larval tis- 



general metabolism, the two types of sues are adapted to live at a low, the 



*This is the conclusion of Dr. Hurley's article on the causes of Amphibian Meta- 

 morphosis. The first part appeared in the preceding number of the Journal. 



