HETEROGONY AND HORMONES 177 



two specificities — the specificity of the hormone, and the 

 specificity of the tissue which reacts to the hormone. For 

 instance, there may or may not be a hormone concerned in 

 the growth of the fiddler-crab's large chela. Should there be 

 one, however, its action would in this case be subordinate to 

 the action of the specific capacity for heterogonic growth 

 possessed by the male type but not by the female type of 

 chela, whereas in a case like that of the fowl's comb, the 

 tissue-specificity is apparently the same (or almost so) in both 

 sexes, and the sexual differences in comb-size are brought 

 about by the specificity of the two sex-hormones. 



So far as we know, there exist no sex-hormones in insects. 

 Accordingly, in this group any secondary sexual heterogonic 

 organs will depend for their development entirely upon their 

 inherent growth-capacities, which differ in the tissues of the 

 two sexes according to the cellular metabolism induced by 

 one or other sex-chromosome complex. In vertebrates, how- 

 ever, the reverse is usually the case : the tissue-capacity is 

 the same or highly similar in both sexes, and the sexual dif- 

 ferences are due to differences in sex-hormones. A good deal 

 of work has been done on the growth of the fowl's comb by 

 Pezard, Benoit, Lipschutz and others (references in Gold- 

 schmidt, 1923) . It appears that at the onset of sexual maturity 

 in males the growth of the comb becomes highly heterogonic, 

 and approximates to a constant differential growth-ratio. The 

 growth of the comb in females is also heterogonic, but mildly 

 so ; while in castrates it is isogonic. The marked comb- 

 changes associated with the onset of a laying period in a 

 pullet may be associated with a change in sex-hormones, or 

 quite possibly with a change in general metabolism. Benoit 

 (1927B) has shown that the growth-coefficient of the comb in 

 growing fowls differs according to the season at which they 

 are hatched. In those hatched in March- June, the coefficient 

 is high early, then decreases markedly in late summer, to 

 resume its high level in October or November ; in those 

 hatched after June, there is no slackening, but the initial 

 coefficient is lower. Benoit suggests that all the phenomena 

 are due to seasonal variations in testis activity. It is prob- 

 able that besides the sex-hormones, many other factors, such as 

 nutrition, influence the growth-partition coefficient of the comb. 



Castration in adult males is followed by a regression in 

 comb-size ; this takes place according to a well-defined 

 mathematical formula. As set forth by Pezard (1921) the 

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