AMPHIBIAN METAMORPHOSIS 615 



it is only the change to frog-organisation that does not occur. 

 In this condition, the statement that the relative size of the 

 thyroid progressively increases as we ascend the vertebrate 

 scale, is of considerable interest. In a perfectly sober sense, 

 the mammal is being continuously drugged (by itself), con- 

 tinuously stimulated to reach a higher level of metabolism 

 than would otherwise be possible, but one which is necessary 

 if the complexity of its organisation is to develop properly. 



In this case we are dealing with balance of glands and 

 tissues, and tissue-sensitisation. Of actual time-relations in 

 development perhaps the most striking example is afforded 

 by the phenomena of puberty, in which again we have an 

 all-or-nothing reaction — the formation of mature gametes, 

 and the full development of secondary sexual characters. 

 It is clear, after the experiments of Steinach (25, 26), Sand 

 (21), Lipschiitz (18), and others, that the gonads in some 

 real sense determine secondary sexual characters : further, 

 Minoura's work (20) has shown that in the fowl, the 

 gonad is producing the same hormonic substance in the 

 embryo, in the chick and in the mature bird, although, in 

 one case, it is determining the accessory sexual apparatus, in 

 the next growth as well, in the last, secondary sexual charac- 

 ters in addition. Why does the determination of full secon- 

 dary sexual characters only occur at sexual maturity ? and 

 why does sexual maturity come when it does ? Interesting 

 unpublished work of Crew's indicates that in various mammals 

 the function of the gonad in determining accessory sexual 

 characters may be, as a not unfrequent abnormality, in abey- 

 ance during the pre-pubertal period, and yet puberty and the 

 determination of full secondary sexual characters take place 

 normally, and at the normal period. This would indicate that 

 the gonad needs activation by different means (presumably 

 by different endocrine organs), in the pre-pubertal and pubertal 

 periods. 



Again, it is often said that glands like the pituitary and 

 suprarenal cortex exert an effect on the onset of puberty. So 

 they certainly appear to do. But what causes them to exert 

 this effect normally at a certain age, and not earlier or later ? 

 That they could exert this effect at other ages is shown by the 

 precocious sexual development caused by tumours in the ad- 

 renal cortex (5), infantilism associated with pineal absence (34), 

 or pituitary hypoplasia, and so forth. That environmental 

 as well as genetic factors play a part is shown by the recent 

 work of Steinach and Kammerer (27), who found that precocious 

 maturity in the rat could be induced by high temperatures, 

 and vice versa. 



Obviously we have here a similar picture to that of am- 



