METAMORPHOSIS IN FROG LARVAE 525 



them upon moist blotting paper; the moist paper prevents 

 laceration of the tail, and at the same time inhibits the occurrence 

 of the swimming reflex, or lashing movements. April 20 the 

 thyroid feeding began. Various methods of administering the 

 thyroid extract were tried. The first method was to place the 

 powdered thyroid in the water and allow the larvae to eat it. 

 This manner of administering the extract soon proved unsatisfac- 

 tory because of the scum which forms over the surface of the 

 water, shutting out the oxygen and causing great mortality among 

 the larvae. Another method which proved eminently satisfactory 

 and was employed throughout the experiment, was to mix the 

 powdered thyroid obtained from Armour and Company with 

 wheat flour, in the proportion of 3 grams of thyroid to 10 grams 

 of flour, with the addition of sufficient water to make a thick paste. 

 The paste was then spread thinly over glass plates and allowed 

 to dry at room temperature. Pieces of the dried paste, the size 

 of one's little finger nail, were finely crumbled and fed to the 

 larvae. The control animals were fed bits of fresh beef liver, 

 in order to equalize the amount of protein material fed the thy- 

 roid animals, and large quantities of algae. 



The thyroid-fed animals ceased to grow after the adminis- 

 tration of the thyroid-flour-paste, and five days from the date of 

 the first feeding, differed considerably in appearance from their 

 controls. The bodies of these thyroid fed larvae appeared 

 slender and the head more elongated than that of the control; 

 all of the larvae revealed indications of tail involution. Eight 

 days after the administration of thyroid, limb buds were observed 

 when the animals were examined with the hand lens. The pig- 

 mentation had increased somewhat in the thyroid-fed tadpoles 

 as they appeared darker than the algae-fed larvae. The elongated 

 appearance of the head, previously noted, had increased; the 

 body had become emaciated and pronounced atrophy of the tail 

 was observed. The limb buds were now plainly visible without 

 the aid of the hand lens. When contrasted with the thyroid-fed 

 larvae, the controls revealed none of the changes enumerated, 

 and had increased in size somewhat, during the eight day interval 

 of the experiment. 



THE JOURNAL OF EXPERIMENTAL ZOOLOGY, VOL. 24, NO. 3 



