6i4 SCIENCE PROGRESS 



and the other tissues is thus a quaUtative, sharply marked 

 response, an end-point. 



It remains to indicate other fields in which similar reactions 

 between developmental processes occur. The best worked out 

 is that of sex-determination. The Morgan school, and Bridges 

 in particular (4), have definitively shown that in Drosophila sex- 

 determination is an affair of balance. The X or sex-chromo- 

 some is predominantly female-determining, the rest of the 

 chromosomes, and especially the small or so-called fourth 

 chromosome, predominantly male-determining. Males, females, 

 ultra-males, ultra-females, intersexes inclining either to male- 

 ness or to femaleness, all can come into existence as a result 

 of different numerical relations of the chromosomes and con- 

 sequent altered equilibrium of genetic factors concerned with 

 sex. 



A still more striking parallel is afforded by Goldschmidt's 

 intersexual moths (11). Certain crosses between the Japanese 

 and European races of Lymantria produce developmental 

 intersexes, i.e. individuals which start as one sex, and after a 

 certain moment change over and continue as the other. By 

 employing different sub-races, the time at which the change 

 occurs is altered, genetic females being in certain crosses all 

 converted into somatically normal males. These phenomena 

 appear to depend upon differences in rate of production of 

 male- and female-determining substances (see fig. 3). We 

 are thus justified in speaking of a sex-metamorphosis in these 

 abnormal individuals and also in protandric or protogynous 

 hermaphrodites, and in comparing the mechanism at work to 

 that operating in Amphibian metamorphosis. A rich field is 

 opened up for investigation into the external agencies which 

 influence the time of this sex-metamorphosis. 



Many similar phenomena can be found in the development 

 of higher forms, although here the conditions are more com- 

 plex, our knowledge less complete. In the first place, the 

 mammal, unlike the tadpole, grows best at a high thyroid 

 concentration. This is well illustrated by Sutherland Simp- 

 son's lambs, which were thyroidectomised shortly after birth, 

 and grew very slowly. In sheep, the psychoneural faculties 

 are httle affected by absence of thyroid ; but in man, con- 

 genital absence of the thyroid gives rise to cretinism, in which 

 not only is growth stunted, but the brain and mind fail to 

 develop. Addition of th3Toid to the diet often cures the con- 

 dition. Thus the mammalian organisation needs for its deve- 

 lopment not only a high uniform temperature, but a high 

 uniform thyroid concentration, and this is more particularly 

 true of the human brain. In the frog-tadpole, on the con- 

 trary, thyroidectomy allows normal growth of every part ; 



