342 C. C. SPEIDEL. 



development of the animal is not adequate to the demands 

 imposed upon it by thyroid treatment and death results. 

 Coupled with this respiratory disturbance is also the condition 

 of anemia already pointed out elsewhere (Jordan and Speidel, 

 '23). The older the tadpole the more probable it is that both 

 forelimbs will erupt. The pulmonary apparatus is presumably 

 better developed, and the forelimbs are large enough so that 

 limb movements aid both in expelling the air from the gill 

 chamber and in breaking through the skin. In thyroid-treated 

 animals that are near the time for metamorphosis both forelimbs 

 are put out with little trouble, the sinistral location of the spiracle 

 becoming of less importance. 



In a recent paper by Swingle ('23) one figure is given to show 

 the effect of iodo-tryosine administration in accelerating meta- 

 morphosis in pituitaryless Rana sylvatica tadpoles. After sixteen 

 days of treatment the specimen illustrated has one forelimb, 

 that one being a right forelimb. Swingle does not state when this 

 particular right limb appeared but does say that two right fore- 

 limbs broke through as early as the eighth day, the average, 

 however, being about twenty days. This is an interesting obser- 

 vation in comparison with my results after thyroid treatment; 

 i.e., 100 per cent, prior left forelimb eruption after the first two 

 days. It seems to mean, either that iodo-tryosine does not 

 affect the respiratory apparatus in the same way as does thyroid 

 extract, or that the pituitary gland plays a role also in influencing 

 symmetrical development. 



It now remains to discuss the normal condition and the 

 thymus-treated condition. Three factors are considered of chief 

 importance in determining forelimb eruption: (i) sinistral loca- 

 tion of the spiracle; (2) relative degree of skin degeneration over 

 the forelimb region on the two sides of the body; (3) relative 

 size and strength of the forelimb. In partly grown thyroid- 

 treated animals the first factor is by far the most important, 

 as has been shown. In mature untreated tadpoles, however, 

 this factor does not remain the all-important one. The fore- 

 limbs are now so large that the left one cannot be pushed through 

 the spiracle without a fair amount of previous skin degeneration. 

 Simultaneous skin degeneration occurs on both sides. The size, 

 strength and activity of the imprisoned forelimb now becomes of 



