THE PHYSIOLOGY OF DEVELOPMENT an 



of thyroxin on the frog tadpole is correlated with an increased 

 respiratory exchange ; and since all the perenibranchiates used 

 for these experiments were of fairly advanced age, it cannot be 

 stated with certainty that these forms are not representatives 

 of the persistent larval stage of species whose '' adult " form 

 has been eliminated through decreased sensitivity to the 

 action of the thyroid hormone. 



These considerations lead us to go back a step and inquire 

 what it is that controls the development and activity of the 

 thyroid in Anura. Light has been shed on this by the work 

 of several investigators of the American school. In Anuran 

 tadpoles the hypophysial rudiment lies above the mouth in 

 a very accessible situation ; by pricking the surface of the head 

 in the embryo at a certain stage the ablation of the pituitary 

 anlage can be accomplished. This was first done by Adler 

 (1914). Smith (19 1 7) and Bennet Allen (19 17) simultaneously 

 and independently discovered that hypophysectomized tadpoles 

 in addition to showing the pigmentary disturbances already 

 mentioned, fail to undergo metamorphosis, this failure being 

 associated with arrested development of the thyroid gland. 

 Later it was shown by Bennet Allen (19 19) that such individuals 

 can be made to complete their development by thyroid 

 administration, and by Swingle (1922) that the same result 

 can be brought about by implantation of the pars anterior. 

 Metamorphosis of the Axolotl by injection of fresh extracts 

 of ox anterior lobe was recorded by Hogben (1922), and meta- 

 morphosis of hypophysectomised frog tadpoles by Smith 

 (1922) (see Spaul, 1925). Smith found that thyroidless tad- 

 poles will not respond to this treatment. On the whole the 

 evidence points to the following sequence : development of 

 the pituitary ; development of the thyroid under the influence 

 of a substance secreted by the pars anterior ; closure of gill- 

 clefts accompanied in Anura by development of limbs and 

 resorption of tail and in urodeles by shedding of the larval 

 skin and resorption of external gills under the influence of 

 the thyroid hormone, with the discharge of which in Urodeles 

 special — at present unknown — agencies are involved. 



Another set of problems connected with the chronological 



