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effect which could still be detected about 60 days after initial treatment. Drasher 

 found that uteri of DBA mice were much more sensitive to estrogen than were the uteri 

 of mice of the C57BL strain. However, when the effects of estrogen on the uteri of 

 these two strains were compared some 70 days after the initial treatment, it was noted 

 that uteri of females of the G57BL strain still showed a carryover effect which manifested 

 itself in a smaller loss in uterine weight, as well as in a smaller loss in the number and 

 size of cells than those noted in the DBA strain. Whether the significantly different 

 rate at which the injected estrogen was metabolized by the two strains is related to the 

 significantly different hospitality of the uteri of these strains noted by Fekete is 

 unknown. 



It seems redundant to cite extensive evidence to show that highly significant 

 differences have been found with regard to the sensitivity of different strains of animals 

 to exogenous hormones. Apparently, degree of inbreeding has little to do with either 

 the sensitivity or with the variability of response. 189 Some strains may be less sensitive 

 to one hormone but more sensitive to another. Munro et al. 920 compared the sensitivity 

 of various end organs of five different breeds of chicks to the various hormones present 

 in a crude anterior pituitary extract (table 56). This preparation contained gonado- 



Table 56 



Order of breed sensitivity to the injection of exogenous hormones (organ 

 weights expressed as percentage of body weight) 



Comb 



Thyroid 



Gonad 



trophic hormones which cause an increase in the weight of testes due to their ability 

 to stimulate growth of the seminiferous tubules as well as of the interstitial tissue. The 

 latter is known to secrete the male sex hormone, androgen, which causes comb growth. 

 In response to gonadotrophic hormone White Rock chicks showed the greatest increase 

 in weight of testes, while White Leghorn chicks were near the bottom in sensitivity. 

 The androgen secreted by the stimulated testes caused the greatest increase in comb 

 size in White Leghorns and an intermediate increase in White Rocks. It is probable 

 that the combs of Leghorns were larger because they were more sensitive to smaller 

 doses of androgen rather than because their testes, in spite of their lower sensitivity to 

 gonadotrophin, secreted more androgen. This illustrates the importance of selecting 

 the proper test animal for each hormone to be assayed. If one wanted to use chicks as 



