132 W. W. SWINGLE. 



very great, even in larvae reared from the same batch of eggs 

 and under similar conditions. Very slight environmental changes 

 are sufficient to bring about fluctuations of the processes of 

 growth in these animals, and even when the changes are reduced 

 to a minimum individual variations are great. I have seen 

 tadpoles of the same age, from the same batch of eggs, reared in 

 the same bowl, that varied as much as 20 mm. in length in five 

 weeks' time. I quote this for what it may be worth, but it 

 serves to indicate the complexity of the factors governing the 

 growth of amphibian larvae. I cannot but think that the results 

 obtained by Professor Gudernatsch are attributable, in part at 

 least, to some other factor besides thymus feeding. 



In regard to the feeding experiments with thymus gland, 

 either in the fresh state or powdered form, one point should not 

 be overlooked, viz., that the growth-accelerating principles of 

 the gland, if^there be such, may undergo disintegration or suffer 

 a change by contact with the digestive enzymes in the alimentary 

 tract. Feeding experiments with thyroid glands show that no 

 such changes occur, but possibly the same is not true of the 

 thymus. 



In conclusion, I wish to acknowledge my indebtedness to 

 Dr. B. M. Allen, the University of Kansas, for the time and en- 

 couragement he has given me in my work. 



BIBLIOGRAPHY. 



1. Adler, L. 



Metamorphosestudien an Batrachierlarven. I. Extirpation endocriner 



Driisen. II. Extirpation der Hypophyse. Arch. f. Entw. mech. d. 

 Organis.. Bd. 39. 



2. Basch, K. 



'05 Uber die Ausschaltung der Thymus. W. K. W., 1903, Bemerkungen 

 zu R. Fischls experimentellen Beitragen zur Frage der Bedeutung der 

 Thymusexstirpation bei jungen Tieren. Z. e. P., II., p. 95, 1905. 

 (Quoted from Biedl, The Internal Secretory Organs.) 



'98 Beitrage zur Physiologic und Pathologic der Thymus. Jahrb. f. 

 Kinderheilk., Bd. 64, 1908. 



'08 Beitrage zur Physiologic und Pathologic der Thymus, II. Uber die 

 Beziehung der Thymus zum Nervensystem. Ibid., 68, 1908. 



3. Biedl, A. 



'13 The Internal Secretory Organs, 1913. 



