HETEROGONY AND HORMONES 



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readily obscure differences due to inherent growth-potential. 

 To detect the latter, it would be necessary to compare the 

 effects on different members or regions of a single organ system 

 — e.g. different bones — which have different growth-ratios. 

 This interesting study at least shows the complexity of the 

 factors by which ' normal proportions ' are determined. 



Here we may cite 

 the paper of Neva- male £ 



lonnyi and Podhrad- 

 sky (1930), which 

 does indicate a differ- 

 ential result of excess 

 thyroid (and of ex- 

 cess thymus) on vari- 

 ous parts of the 

 skeleton of fowls. 

 Unfortunately the 

 thyroid results are 

 based on only two 

 experimental and 

 four control animals, 

 so that confirmation 

 is required. It would 

 appear, however, 

 that on the whole 

 there is a tendency 

 for excess thyroid (in 

 this dosage) to pro- 

 duce relatively 

 thicker bones ; fur- 

 ther, to encourage 

 the total growth of 

 the femur, shoulder- 

 girdle and hip-girdle, 

 but to decrease the 

 1 e n g t h-growth at 

 least of the meta- 

 carpals and metatarsals ; the radius, ulna and tibia occupying an 

 intermediate position. This is evidence, so far as it goes, of 

 a graded centrifugal effect. We may also note that Hammett 

 found femur less sensitive to thyroid defect than humerus, 

 and the Czech authors found it respond more by excess growth 

 to thyroid excess. 



Fig. 83. — Effect of thyroidectomy on the growth 

 of the hypophysis in the male Albino rat. 



The graph is constructed as described in Fig. 82. The hypo- 

 physis is the only organ which increases more in hyroidec- 

 tomized than in Control animals. 



