Huxley: Glands and Development 



stantiated, the change of environment 

 which appears to initiate metamorpho- 

 sis would here be external, not internal. 



To revert, however, to our special 

 problem of Amphibian metamorphosis. 

 We have seen that the consecutive 

 dimorphism of metamorphosis is com- 

 parable to that of certain types of her- 

 maphroditism. It remains to point out 

 that, if such were of advantage to the 

 organisms, it would be easy, theoret- 

 ically, to establish a simultaneous di- 

 morphism similar to that of bisexual 

 organisms. If in an animal like the 

 Axolotl, for example, in which the 

 balance between thyroid and other tis- 

 sues is fairly close, a mutation were 

 to occur that would result in bringing 

 about metamorphosis, then if we call 

 this the mutant factor T and the nor- 

 mal factor t. we should have meta- 

 morphosing animals of classes TT 

 and Tt and neotenous individuals of 

 class tt. It this condition were ad- 

 vantageous, it could no doubt be per- 

 petuated, as is the dimorphism of 

 Primula in regard to style-length. As 

 a matter of fact, it is impossible to 

 imagine that such a condition could be 

 of biological advantage : none the less 

 the illustration is useful as showing 

 the very close connection which exists 

 between the mechanism of metamor- 

 phosis and that of sex-dimorphism. I 

 will not press the point ; those inter- 

 ested can work out for themselves com- 

 parisons with gynomonoecism and 

 various other kinds of sex-distribution. 



A further interesting parallel with 

 sex is provided by the unpublished ob- 

 servations of Dr. Hogben, to whom 

 many thanks are due for permission 

 to mention them. Certain combina- 

 tions of anterior and posterior pitu- 

 itary treatment, it seems, lead to the es- 

 tablishment of an apparently permanent 

 state intermediate between the larval 

 and the adult form, and comparable to 

 the balanced triploid intersexes of 

 Bridges' Drosophila ( in Science. Xew 

 Series 54:-252, 1921). 



Other Factors 



The ]n-oblem has thus far been sim- 

 plified by omitting almost completely all 

 references to other factors, which, like 

 the pituitar}-. ha\'e an effect uix)n meta- 

 morphosis.-- "■ ^"' 1"^ 



These, however, seem usually to 

 produce this effect indirectly, by act- 

 ing upon the thyroid, and can, there- 

 fore, for the purposes of an essay such 

 as this, be logically included under the 

 general heading "rest of the body," or 

 the "general larval organization." 



It is clear, however, that the "gen- 

 eral organization" is very complicated, 

 and that its effect on metamorphosis 

 is theoretically separable into a num- 

 ber of single factors, all of which can 

 be considered as coming into a relation- 

 ship, whether positive or negative, with 

 the effect of the single factor of thy- 

 roid-differentiation. In precisely sim- 

 ilar fashion, the work of Bridges shows 

 that as regards sex we have on the one 

 hand the single factor (using the word 

 not in its technical but in its general 

 sense) of the sex-chromosome which 

 may be present in single or in double 

 dose, and on the other hand the opera- 

 tion of a number of factors, namely 

 individual genes scattered throughout 

 the autosomes, some exerting effects 

 antagonistic, others effects comple- 

 mentary to that exerted by the sex- 

 chromosome. 



It is clear that the more we know 

 al)out these individual factors, the 

 further will our analysis proceed. 

 What I would specially like to point 

 out is this : metamorphosis provides a 

 clearly defined and easily recognizable 

 reaction, which throws light on certain 

 important processes occurring in the 

 organism ; it may in this respect be 

 compared with the visible reaction of 

 litmus or other indicators to changes in 

 hydrogen-ion concentration. 



The Physiology of Development 



But what is of special interest, if the 

 views that I have been expressing are 

 well-founded, is that the phenomena 



