No. 438.] 



AMBL Y STOMA TIGRINl J/. 



397 



One only had proved refractory, not having even begun the 

 change. This was one from which the gills had been removed. 

 Like all the specimens it had thriven and made good growth, yet 

 its gills had grown to barely one third natural size. At this 

 time the specimen was utilized for an experiment under running 

 water, and, despite its several weeks of life with practically no 

 gills, and the small size at which these organs still remained, it 

 withstood fairly well the confinement under water, dying at the 

 end of the second day just after metamorphosis had begun. 



The foregoing facts, together with many more like them, have 

 led me to think that the acceleration or retardation of metamor- 

 phosis in our species of Amblystoma, is little, if at all, a question 

 of enforced air-breathing, of gill development, of oxygenated or 

 unoxygenated water. Is it then a question of temperature or of 

 light ? Again the answer must be largely negative. Not that 

 the writer would deny to these important agencies all influence. 

 A very low temperature checks all life activities in Amphibia ; 

 metabolism sinks to the lowest ebb, and metamorphosis is 



early metamorphoses are not produced chiefly by excess of light 

 or heat; while delayed metamorphosis is certainly not alone the 

 result of darkness and low temperature. I will mention the fol- 

 lowing under light : 



Larvae sometimes attain great size in Nebraska ponds despite 

 their complete exposure to the relatively constant sunlight of our 

 clear summer climate. In the laboratory, the largest larva I have 

 ever reared — 17.5 cm. — was kept in a rather small battery jar 

 exposed to the full light of an east window. Others in the same 

 window metamorphosed at very varying stages ot growth. On 

 the other hand, larvae in dark aquaria have frequently meta- 

 morphosed with the utmost readiness. In a long series of 

 experiments on the causes of color variation, larva' were exposed 

 for weeks before metamorphosis to all possible degrees of light 

 and darkness (not to lights of the different primary colors) and 

 no obvious retardation or acceleration of metamorphosis resulted. 



