318 THYKOID AND IODINE AND METAMORPHOSIS 



OBSERVATIONS AND TABULATION OF DATA: 



There is high mortality in this type of an experiment. 



Mount some millimeter graph paper on the underside of some flat-bottomed Petri dishes 

 to use as a guide in determining the size changes in tadpoles. The important criteria are: 



a. Hind limb length 



b. Total length, i. e. , from snout to tip of tail 



c. Body length, i. e. , from snout to base of tail 



Other items to note will be changes in the mouth, the shape and size of the head, the ap- 

 pearance of the forelimbs, resorption of the tail, first shedding of skin, formation of 

 the tympanum, and finally relative dehydration, etc. By means of averages of these size 

 changes, determine the body proportions that show maximum response to the thyroid. 



While the tadpoles are exposed to the thyroid or iodine treatment for only a short time 

 (1 to 2 weeks) the final data should not be collected for from 1 to 2 weeks (or more) 

 after the cessation of the experimental conditions. Some of the tadpoles should be kept 

 until they achieve metamorphosis and the time of emergence from the water of the ex- 

 perimentals and parallel controls should be noted. Normally Rana pipiens larvae will 

 reach stages of metamorphosis in about 75 days after the eggs are fertilized, if kept at 

 laboratory temperatures of 23 -25 C. , fed well, and not crowded. 



Arrange your data in tabular form and illustrate with drawings or photographs. 



DISCUSSION: 



It is not within the province of this Manual to carry an endocrine study beyond the macro- 

 scopic examination. The relation of the thyroid and/or the pituitary gland to metamor- 

 phosis has been the subject of long and thorough studies. There are, however, other 

 embryological approaches to the problem which might bear investigation. These are 

 suggested in the form of questions, as follows: 



1. Will thyroid or iodine affect stages of development prior to the limb-bud stages, 

 or stages prior to the normal functioning of the host thyroid? Is there any effect 

 on the blastula or gastrula or neurula, for instance? 



2. The pituitary is known to have thyreotropic function, but which of these glands is 

 the ontogenetic precursor? 



3. What would be the effect on the embryos (larvae) following extirpation of either or 

 both the thyroid and the pituitary anlagen? 



4. What is the effect on them of grafting thyroids (or pituitaries) of the same and of 

 advanced ages into the tail blastemas of tadpoles. 



5. What is the effect of thyroid and of thyroidectomy upon the regenerative processes 

 of the larva ? 



REFERENCES 



ADAMS, A. E. , 1941 - "Studies in experimental zoology. " Edwards Bros. Ann Arbor, Mich. 



ADOLPH, E. F. , 1931 - "Body size as a factor in the metamorphosis of tadpoles. " Biol. Bull. 61:376. 



ALLEN, B. M., 1938 - "The endocrine control of amphibian metomorphosis. " Biol. Rev. 13:1. 



BOWER, CM., 1938 - "Growth rates of the hind limbs of Rana sylvatica during normal and induced metamorphosis. " Anat. 



Rcc. 72:suppl. 99. 

 BOWERS, C. Y. , A. SECALOFF G B. BROWN, 1959 - "Factors affecting the thyroid gland uptake of l'^' of the Rana catesbiana 



tadpole." Endocrln. 65:882-888. 



