AMPHIBIAN METAMORPHOSIS 617 



holism continues in the absence of the thyroid — but it drops 

 about 60 per cent. Some ossification occurs in the old Axolotl, 

 but takes years to reach the same general degree which an 

 overdose of thyroid will bring about in three weeks. Growth 

 and differentiation occur in the limbs of thyroidless tadpoles, 

 but at a rate perhaps 30 or 40 times less than in normal animals, 

 and 300 or 400 times less than in thyroid-fed specimens. 



It is useless to pursue the subject further in the present 

 state of our ignorance ; but I hope that I have said enough 

 to show that, if real accuracy of experimentation is introduced 

 into such a subject as that of amphibian metamorphosis, and 

 if we accustom ourselves to thinking in terms of rates and 

 equilibria and processes, we shall in a few years be able to lay 

 the foundations of a branch of biology which will bring the 

 same sort of view-point into the whole subject as was intro- 

 duced into chemical science by the study of physical chemistry. 

 We are now, thanks to the labours of the physiologists, learn- 

 ing to appreciate the delicacy of regulation in the adult ; 

 we shall then embark on the study of regulation in the deve- 

 loping organism. 



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REFERENCES \ 



Abelin, 1920. Biochem. Ztsch., 102, 58. 



Abler, 1916. Pfliiger's Arch., 164, i. 



Barfurth, 1887. Arch. mikr. Anat, 29, i. 



Bridges, 192 i. Science, 54, 252. 



Bullock and Sequeira, 1905. Trans. Path. Soc, 56. 



Champy, 1922. Arch. Morph. Exp. et Gen., 1, no. 4. 



Gushing, 19 12. The Pituitary Body and its Disorders. Philadelphia 

 and London, 1912, 



Dumeril, 1876. Ann. Sci. Nat., (5) 7. 



Gilford, H., 191 i. The Disorders of Post-natal Growth and Develop- 

 ment. London, 191 1. 



Gilford, 1913. Lancet, 1913, 587 and 664. 



GoLDscHMiDT, 1920. Ztsch. ind. Abst. Vererh., 23. 



GuDERNATSCH, 1912. Arch. Entw. Mech., 35, 457. 



HoGBEN, 1922. Proc. Zool. Soc, 1922. 



HosKiNS, 1920. Endocrin, 4, i. 



Hegner, 1922. Am. J. Physiol., 61, 298. 



Huxley and Hogben, 1922. Proc. Roy. Soc, 93 (B), 36. 



Kendall, 1919. Am. J. Physiol., 49 (Proc), 136. 



LiPSCHUTz, 1919. Die Pubertatsdriise und ihre Wirkungen, 1919. 



M'Carrison, 1917. The Thyroid Gland^ London, 1917 (p. 29). 



MiNouRA, 192 1. /. Exp. Zool., 33, i. 



Sand, 1921. J. de Physiol, et Pathol. G6n., 19, 305. 



Smith, P. E., 1920. Amer. Anat. Mem., no. 11. 



Smith and Smith, 1922. Anat. Rec 23 (Proc), 38. 



Starling, 1920. Principles of Human Physiology, 3rd Ed., London, 

 1920 (p. 696). 



Steinach, 1913. Centralbl. f. Physiol., 27, no. 14. 



Steinach, 1920. Arch. Entw. Mech., 46, 12. 



(27) Steinach and Kammerer, 1920, Arch. Entw. Mech., 46, 391. 



