526 W. W. SWINGLE 



Twelve days after the first thyroid extract was fed, the larvae 

 of both thyroid-fed and control cultures were measured. The 

 average length of the experimental larvae was 7 mm., of the 

 controls 16 mm. The tadpoles fed with thyroid had diminished 

 in length 3 mm. ; the normal larvae had increased in length 6 mm. 

 The differences between the two series of animals were very great. 

 The tails of the thyroid-fed larvae had shriveled greatly, and in 

 three larvae had been almost completely resorbed. Many of 

 the larvae experienced great difficulty in swimming because of 

 their rudimentary tails. The hind limbs were very noticeable, 

 though as yet they had not completely differentiated into their 

 two primary divisions. Other indications of metamorphosis 

 were apparent: the sucker-like tadpole mouth had begun to 

 assume the shape typical of that of the adult frog; the elongated 

 appearance of the head was found, upon examination of the 

 viscera, to be due to a remarkable atrophy and resorption of the 

 long coiled gut. 



The intestines of thirty of these thyroid-fed larvae and thirty 

 of the normal animals were measured; the entire length of the 

 gut was included, from the oesophagus to the anus. In the 

 thyroid-fed animals the average length of the alimentary tract 

 was 20.5 mm., whereas in the controls the average length of the 

 gut in thirty larvae was 50.5 mm. The appearance of the gut 

 of the thyroid-fed larvae differed remarkably from that of the 

 controls in that it was very like the alimentary tract of the adult 

 form. The gut no longer presented the appearance of a long 

 coiled tube of about equal diameter throughout its entire length; 

 instead, the typical frog stomach and large and small intestine 

 had differentiated. 



Besides these changes in the viscera, there appeared to be 

 certain marked growth changes in the bones of the heael. These 

 changes, however, were not investigated. 



The mortality among the thyroid-fed larvae became very 

 great after the twelfth day of thyroid feeding, Each morning- 

 four or five larvae were found dead in the containers. 



May 5, or just fifteen days from the date the experiment began, 



