On Amphibian Metamorphosis and Internal Secretions. 41 



(S) was distinctly less sluggish than the other two, between which not much 

 difference could be noticed. 



The experiment was not wholly conclusive. The differences between the 

 rates of metamorphosis of the four thyroid-fed animals was very slight. 

 There was, however, a slight retardation seen in the specimens kept in 

 alcohol, and a retardation which increased with the concentration. 



The experiment was therefore repeated with the following variations : 

 (1) Control : meat-fed in water. (2)-(5) Thyroid-fed : (2) in water ; 

 (3) in 1/3 per cent, alcohol; (4) in 2/3 per cent, alcohol; (5) in 1 per 

 cent, alcohol ; two animals in each dish. The same slight retardation of 

 metamorphosis with alcohol, increasing with concentration, was observed. 

 Again, larger numbers are really necessary before the retardation can be 

 regarded as proved ; but, so far as it goes, these experiments appear to be 

 significant. 



All the alcohol specimens this time showed redness and congestion of the 

 gills, increasing with concentration. The congested gills were not reduced 

 till relatively very late, and then decreased remarkably rapidly in size. 

 The 2/3 per cent, and 1 per cent, specimens all died suddenly just before 

 they were due to metamorphose. Metamorphosis in the control took 

 29 days ; in the 1/3 per cent. 29 and 31 days. 



(d) Size of Thyroid. 



From all recent work upon the metamorphosis of Amphibia, it would 

 appear certain that metamorphosis is normally associated with thyroid 

 activity, taking place when certain substances produced by the gland reach 

 a definite concentration in the body. In those forms, therefore, in which a 

 typical metamorphosis rarely or never occurs, we should a priori expect to 

 find an abnormally small thyroid. 



In the Axolotl, however, the thyroid is not abnormally small ; further, 

 it presents a perfectly normal histological picture. It would only be 

 possible to give definite data after a careful examination of the relation 

 between thyroid-weight, body-weight, and, if possible, iodine-content (or other 

 criterion of activity) of thyroid in a number of Urodela, including normally 

 metamorphosing forms, the Axolotl, and some Perennibranchiates. There is, 

 however, one observation which it is of interest to mention here. That is 

 the abnormally large size of the thyroid in Siren. This has been recorded by 

 Wilder in his anatomical account of the animal (1891); and his account we 

 have confirmed from the dissection of two specimens preserved in spirit in the 

 Oxford University Museum. The thyroid is not only as large as in a normally- 

 metamorphosing species, but much larger. Of this fact there can only be 



