238 GENETICS 



might not be provided in some other way. Could they be 

 introduced into the body with the food, or by injection into 

 the blood? If so, the characteristics produced would be 

 controlled from outside. Or can the hormones be modified 

 or removed, in such a way as to change the characteristics 

 of the individual? 



As before seen (Chapter III) this can indeed be done. 

 The hormone produced by the male embryos (those having 

 the single X-chromosome) may be transferred to the em- 

 bryo that would otherwise produce a female, and cause it 

 to produce male characteristics. In a similar way, the action 

 of an imperfect thyroid hormone can be supplemented from 

 outside. Individuals in which the genes supply a poor thy- 

 roid hormone fail to develop normally; in man they form 

 the cretin, whose mental development is stunted. But this 

 lack may be supplied by introducing the thyroid hormone 

 with the nutrition; thereupon the development returns to 

 its normal course. 



In these cases products of the genes are transferred from 

 one individual to another, where the same effect is produced 

 as would result if the substance came from the operation of 

 the individual's own genes. But a further step is possible. 

 In some cases the chemicals controlling development are 

 produced artificially and introduced into the developing 

 body, where they produce the same effect as those pro- 

 duced by the organism itself. The active principle of the 

 thyroid has been synthesized, and this synthetic product 

 may be used in place of that produced by the genes, with 

 the same effect on the development of the individual. Prog- 

 ress has been made in producing artificially other products 

 of the genes, such as epinephrin, the secretion of the supra- 

 renal body, and insulin, the internal secretion of certain 

 cells of the pancreas. The production and use of hormones 

 or endocrine secretions, and the control of development or 

 function through their use, has become a vast chapter of 



