ENDOCRINE GLANDS AND BILATERAL SYMMETRY. 34! 



in. But apparently the animal is not yet very well fitted for 

 utilizing it properly. Small bubbles of air collect in the gill 

 chambers and are not expelled, or expelled with difficulty. This 

 is especially true of the right gill chamber which has no outlet 

 except through the left spiracle (and, of course, the mouth). 

 This proves to be the deciding factor. The left gill chamber is 

 usually drained well enough except in very immature tadpoles, 

 so that skin degeneration and forelimb eruption on that side an 

 not interfered with. On the right side, howe\er. the retention 

 "t air in the chamber causes bulging out of the skin I ig. :?>. and 

 interferes to a greater or less degree with the normal eruption 

 on that >ide. thus bringing about the typical prior left forelimb 

 eruption. A tadpole in this stage does not present a normal 

 uire \\hen at rest, but floats with the right side somewhat 

 ele\ated owing to the air in the right gill chamber. The- dil'ler- 

 enre between a thyroid-treated animal in this condition and 

 either normal or thymus-treated animals may be seen by rom- 

 oarini; I i'<. j with Figs. $ and 4. Occasionally, in very v 





m 



3- 4- 



I- IG. 3. Dorsal view of a normal tadpole (Hyla a l>cfore the eruption 



i rithri i..M-liml'. 1 In- location of the spiracle (5) on tl Ic only. ]>-.i-U t.. 



a majority (70 per o left forelimb eruptions. 



1- ic. 4. Dorsal view t a thymus-treated tadpole (//v/<j crucifer) showing pri<>r 

 right forelimb eruption. Eitln-r right or left forelimb may erupt first. In the 

 region of the spiracle (5) may be seen an angular projection (/>) caused by the 

 pressure of the imprisoned Icit i.-relimb. 



ami immature tadpoles no forelimbs erupt with thyroid treat- 

 ment, the animals dyiiu in the two-limb stage. In these, bubble- 

 of air may be seen in both gill chambers. The pulmonary 



