10 



The Journal of Heredity 



for which metamorphosis acts as an 

 indicator are processes of differentia- 

 tion and that it is their relative rates 

 which are important. We are intro- 

 duced to a possibiHty of studying by 

 experimental means a field which so 

 far has scarcely been accessible save to 

 observation — I mean the time-relations 

 of development. So far, the science of 

 Developmental Physiology has sought 

 to establish simple causal relations be- 

 tween particular developmental happen- 

 ings within or without the organism. 

 It has established the optimum, mini- 

 mum, and maximum temperatures for 

 the development of many organisms, 

 the chemical substances necessary for 

 growth, the abnormalities produced bv 

 alterations in the medium. It has 

 further found that certain organs may 

 exert "formative stimuli." to use 

 Herbst's well-known phrase, upon the 

 development of other organs. Clas- 

 sical examples of these are the influence 

 of the optic cup of vertebrates upon 

 lens-formation, and the influence of 

 the optic ganglion and ant2nnary centre 

 of Crustacea upon the form of the 

 structure regenerated after the eye- 

 stalk is cut ofif. 



Rut De\-elopmental Physiology can 

 hardly be said to have entered upon its 

 quantitative phase. To do this, it is 

 essential that we think in terms simi- 

 lar to those introduced into chemical 

 .science by the methods of Physical 

 Chemistry. We must think in terms of 

 processes and relations between pro- 

 cesses. The physiology of development 

 is quite dilTerent from the physiology 

 of adult life, since the latter is mainly 

 concerned with the regulation of the 

 functions of the body so that thev do 

 not depart beyond certain limits from 

 the normal. Developmental Physiology, 

 on the other hand, is concerned, not 

 with constancy, but with change ; dur- 

 ing develo])ment continually new struc- 

 tures and functions are appearing, each 

 pha.se proceeding to a ])oint at which 

 it resolves itself into a new and quali- 

 tatively different ])hase ; and the regu- 

 lation that exists is concerned with the 



proper sequence of these phases in such 

 a way that a normal adult is the out- 

 come. 



Thus if Entwickhmgsmcchanik, or 

 Developmental Physiology, wishes to 

 become accurate, it must concern itself 

 with the speed and the equilibrium of 

 processes. Time-relations, on the one 

 hand, interaction on the other — these 

 are the two chief points with which 

 it must deal. We have seen that 

 metamorphosis depends upon the time- 

 relations, inter alia, of thyroid-differen- 

 tiation ; we have also seen that the 

 inter-relation of thyroid difi^erentiation 

 with other developmental processes has 

 to be considered. This point of view has 

 been introduced into general biology 

 mainly by Goldschmidt's work on con- 

 secutive intersexuality. It is, I think, 

 safe to prophesy that Amphibian met- 

 amorphosis will provide the most fa- 

 vorable field for the next steps in its 

 cultivation. There are many clinical 

 observations on mammials and man 

 which await elucidation from such a 

 quantitative study of Developmental 

 Physiology: variations in the onset of 

 puberty ; pathological sexual precocity ; 

 infantilism; progeria; premature senil- 

 ity and so forth. 



In conclusion, one or two points may 

 be touched upon which these general 

 considerations help to illuminate. We 

 have seen that it is a fallacy to think 

 of the thyroid as having a specific 

 metamorphosis-producing effect. In the 

 same way, it is a fallacy to think of it 

 as being a specific growth-promoting 

 agent. This idea has gained currency 

 owing to the fact that cretins are per- 

 manently stunted in growth, and that 

 early thyroid administration often 

 causes normal growth to be resumed. 

 A physiologist with whom I was con- 

 versing recently was much surprised — 

 one could almost say horrified ! — on 

 hearing that thyroidectomy did not ad- 

 versely afi^ect the growth of tadpoles. 

 The fact of the matter appears to be. 

 as we have seen, that certain tissues 

 grow well at a low level of thyroid 

 concentration, others at a high level. 



