Huxley: Glands and Development 



11 



Among those which appear to need 

 a high level, those composing the 

 human brain are prominent. Many of 

 the stunting effects of cretinism are 

 quite probably secondary, due to the 

 absence of the normal trophic effects 

 of the stunted nervous system; (com- 

 pare the experiments of Forsterling on 

 similar secondary stunting due to x-ray 

 treatment of the brain region in mam- 

 mals). In a precisely similar way, the 

 human brain requires a high tempera- 

 ture to work properly; lowering its 

 temperature a few degrees will bring 

 about a failure of consciousness. Sim- 

 ilarlv, it is so familiar as often to 

 escape notice that a high temperature 

 and consequent general rapidity of 

 metabolism is necessary for develop- 

 ment to occur at all in mammals and 

 birds ; their development ceases at a 

 temperature still above the optimum 

 for that of Amphibia. 



In this connection, it is interesting to 

 note that the relative size of the thy- 

 roid is stated to increase progressively 

 in the different classes of Vertebrates, 

 being smallest in fish, and largest in 

 warm-blooded animals. To put it as 

 crudely as possible, "frog-constitution" 

 without thyroid only becomes a tad- 

 pole ; "man-constitution" without thy- 

 roid becomes a cretin. The difference 

 between the two cases is that one is 

 pathological, the other is not. This 

 difference between the requirements of 



the tissues of tadpoles and those of 

 warm-lilooded Vertebrates should not 

 be forgotten when estimating the value 

 of experiments with tadpoles as a 

 means of throwing light on human 

 physiology. 



Summary 



(i) Amphibian metamorphosis is 

 considered as a transition between two 

 phases in a consecutive dimorphism, 

 and as initiated by a change in internal 

 environment. 



(2) The similarity of metamorpho- 

 sis to the sex-change in protandric and 

 protogynous hermaphrodites and in 

 Goldschmidt's intersexual moths is em- 

 phasized ; the dimorphism of sex is 

 associated, like that of metamorphosis, 

 with differences in internal or external 

 environment. 



(3) The similarity of the break- 

 down of larval tissues to that of the 

 tissues of lower organisms in dediffer- 

 entiation and resorption is pointed out. 



(4) Amphibian metamorphosis is 

 probably determined by the relation 

 between thyroid differentiation and 

 the processes involved in the general 

 metabolism of the rest of the larval 

 body. 



(5) Through the study of Am- 

 phibian metamorphosis, we are enabled 

 to study the processes of differentiation 

 quantitatively as regards their individ- 

 ual rates, and their interaction with 

 each other. 



I. 

 2. 



3- 

 4. 

 5. 

 6. 



7. 

 8. 



9- 

 10. 

 II. 

 12. 



i;;. 



14. 

 15. 



16. 



17. 

 18. 

 19. 

 20. 



Allen, B. M. 



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