340 



C. C. SPEIDEL. 



the left limb is pushed through the spiracle, a variable amount of 

 preliminary skin degeneration occurring. The right forelimb 

 erupts through the skin only after the latter has undergone a 

 certain amount of degeneration. This degeneration starts in the 



FIG. i. 



FIG. 2. 



FIG. i. Ventral view of a tadpole of the tree frog (Hyla crucifer) showing well 

 developed forelimbs ()) still imprisoned beneath the skin in the gill chambers 

 (r.g.c. and l.g.c.). The position of the sinistral spiracle (s) is shown, which drains 

 both gill chambers; also the canal (c) across the mid-line which connects right and 

 left gill chambers. The dotted line below the forelimbs indicates the position of 

 the partition separating the gill chambers from the abdominal cavity. 



FIG. 2. Dorsal view of a thyroid-treated tadpole (Hyla crucifer) in typical 

 three-limb stage after the eruption of the left forelimb. Collection and retention 

 of air (o) in the right gill chamber causes bulging out of the skin and interferes 

 with forelimb eruption on that side. Excluding the first two days, prior left 

 forelimb eruption occurs invariably. 



vicinity of the elbow, and the elbow is usually protruded first. 

 Movements of the forelimb finally enable it to break completely 

 through. In young thyroid-treated tadpoles, however, the fore- 

 limbs are little developed so that the elbow is not prominent at 

 the surface. In these cases the hand or whole arm appears as 

 a tiny white stump after the skin degeneration has proceeded 

 far enough. 4 



The administration of thyroid extract to a half-grown tadpole 

 obviously upsets the respiratory mechanism. More air is taken 



'While this paper was in press, Helff ('24), at the Washington meeting of the 

 American Society of Zoologists, reported a series of experiments which show clearly 

 that the opercular skin autolysis preceding forelimb eruption is brought about by 

 substances given off by the adjacent atrophying gills. 



