534 W. W. SWINGLE 



whole, however, the gonads of the two series of animals appeared 

 to be in the same stages of development. 



No microscopic examination of the germ cells was made. 



September 23, a second series of Rana catesbiana larvae was 

 started upon the thyroid diet — appropriately controlled. The 

 method of administering the extract was the same as that al- 

 ready described. The larvae averaged in length, September 23, 

 75 mm., most of the tadpoles had small hind limb buds, though 

 as yet they had not developed into their two primary divisions 

 nor had the toe points differentiated. 



October 10, the thyroid-fed tadpoles exhibited all the symp- 

 toms of hyperthyroidism characteristic of these animals. The 

 tails of the larvae were somewhat atrophied and frayed along 

 the edges; the heads appeared elongated; the bodies were darkly 

 pigmented and greatly emaciated. All of the larvae had large, 

 well developed hind limbs, averaging in length 11.5 mm. In 

 some of the animals, the fore legs had broken through the skin 

 and though very small, they seemed well formed. The tadpole 

 mouth showed signs of breaking down to form the mouth char- 

 acteristic of the adult. The urostyle was plainly visible on all 

 of the thyroid-fed animals. As is obvious from the description, 

 all of the experimental animals were well along in metamorphosis. 



October 16, the developmental stages enumerated for the 

 animals of the thyroid-fed group were very pronounced. The 

 tails of all the larvae were undergoing resorption. In two of the 

 animals, the tails were mere stubs, the animals having completed 

 metamorphosis. The mortality among the animals became so 

 great at this time that it was considered advisable to kill all but 

 two of the thyroid-fed tadpoles, together with a similar number 

 of the controls, and preserve them for microscopic examination 

 of the gonads. 



When killed, most of the animals had nearly completed meta- 

 morphosis, whereas the control animals had increased in size 7.5 

 mm. during the interval the experiment continued. There were 

 no indications of metamorphosis in the algae-fed controls. In 

 figures 6 and 7 are shown the thyroid-fed and control animals 

 of this group. The developmental differences between thyroid- 



