46 



Messrs. J. S. Huxley and L. T. Hogben 



Original length. 



Increase in length in 31 days. 



Average. 



Average. 



Eange. 



mm. 



mm. 



mm. 



A. Thyroid diet 



B. Pituitary diet 



C. Alternate thyroid and pituitary 



63 -0 

 62 

 62 -5 



1-5 

 15 -0 

 4 5 



-5-3 -0 

 14 0-16 -0 

 3 -0-6 -5 



diet 

 D. Meat diet 



61-5 



13 -0 



10 "0-17 -0 



It will thus be seen that thyroid diet in this experiment did not cause a 

 diminution of size, but permitted a very small increase. The controls fed on 

 meat grew rapidly, but their growth was definitely, if slightly surpassed, by that 

 of the pituitary-fed animals. Those fed alternately on pituitary and thyroid 

 showed only a small amount of growth, but it was almost three times that 

 recorded for those fed on thyroid alone, whereas the proportion of delay of 

 metamorphosis in (C) was by no means equally great. From this, as well 

 as from (B), it would appear that pituitary has definite growth-promoting 

 properties for Amblystoma larvae. 



Half-a-dozen animals used in the pituitary-feeding experiments were taken 

 at intervals of time and transferred to thyroid diet. The time taken for 

 metamorphosis in three cases was slightly longer (33 to 35 days to 

 emergence) than in controls, so that there may have been a retardation. In 

 any case, these larvae were clearly able to complete their metamorphosis.* 



One individual was also fed for 3 months on fresh prostate without 

 showing any perceptible signs of transformation, though subsequent thyroid 

 treatment induced metamorphosis. 



The suggestive, but inconclusive, experiments made by Jensen, with a 



view to elucidating more fully the significance of the Perennibranchiate 



condition in relation to the physiological processes underlying metamorphosis 



* One animal was continued on the pituitary diet for some time, a control in identical 

 conditions being fed on ox muscle. After five months, the pituitary-fed specimen 

 weighed 32"5 grm., the control 11'2 grm. Other animals of the same age fed on meat 

 were all of about the same size as the control. It has since been suggested to me that 

 if the control had been fed on more succulent diet, such as brain or liver, it would have 

 rivalled the pituitary-fed specimen. Until the suggestion can be tested experimentally, 

 I content myself with stating the facts. The growth of meat-fed specimens in the 

 previous experiment, lasting 31 days, was about as great as that of the pituitary-fed 

 animals. Later, the meat-fed animals' growth became relatively much slower. Another 

 noticeable feature was that in thyroid-metamorphosed specimens previously fed on 

 pituitary the size of the limbs was greater than in those not so treated. — J. S. H. 



4. Effect of Thyroid-feeding on Necturus. 



