THE FUNCTIONAL PERIOD OF DEVELOPMENT 



427 



thyroid throwing its stored secretion into the blood. ^ In this re- 

 spect, the Urodele may be contrasted with the Anuran, where the 

 thyroid becomes progressively more active during larval life, with- 

 out any such extreme change in its activity. 



The morphogenetic effects of hormones are varied. Some of the 

 most marked are those concerned with amphibian metamorphosis, 

 in which the growth or differentiation of some organs and the 







Fig. 207 

 Sharply delimited fields in a thyroid-treated frog tadpole. Section showing on 

 the left the epidermis of the fore-limb bud, on the right the lining of a branchial 

 cleft. The former has reacted to the thyroid hormone by growth (mitoses, 

 crowded nuclei); the latter by degenerative changes (vacuolation, shrunken 

 nuclei). The limit (/.) between the two zones is clear-cut, without transition. 

 (From Champy, Arch. Morph. Gen. Exp. iv, 1922.) 



atrophy of others will only take place under the influence of the 

 thyroid hormone. All gradations are to be found, however, be- 

 tween such marked morphogenetic effects and effects of a transitory 

 physiological nature. The morphogenetic effect of hormones 

 may be linked with the pre-existence of qualitatively different 

 fields. E.g. in the frog, one region of epidermis will proliferate, 

 and another degenerate, under the influence of thyroid- (fig. 207). 

 1 See Huxley, 1923; Uhlenhuth, 1922. ^ Champy, 1922. 



