HETEROGONY AND HORMONES 183 



fowls. The annexed table, modified from his Table 5, shows 



the chief results. 



TABLE XIIa 



Percentage Change in Size in Parts of the Limbs of Male Fowls 



Induced by Castration 



Carpo- Phalanges of 



Humerus Radius, Ulna metacarpus digit 3 (mean) 



Fore-limb . + 2-6 +2-25 +2-2 — o-i per cent 



Hind-limb . +3-0 +3-4 +3-9 +4 -7 per cent 



Femur Tibio- Tarso- Phalanges of 



tarsus metatarsus digit 3 (mean) 



It will be seen that castration causes in general an increase 

 in the size of the limb-bones. But whereas in the hind-limb 

 the increase itself increases as we pass distally, in the fore- 

 limb it is graded in the reverse sense, leading to an actual de- 

 crease in the terminal segment. The existence of the gradients 

 is interesting, but the explanation of their opposite sign in 

 fore and hind limbs is at present quite obscure. 



Hammett (1929B) has published a summary of our knowledge 

 of the effect of thyroid upon growth. In the first place, the 

 effect upon total growth is an affair of dosage. In intact 

 young mammals (and probably many other vertebrates) slight 

 excess of thyroid causes an increase of growth in weight, while 

 heavier doses, by a differential encouragement of katabolism, 

 reduce it. In thyroid-deficient animals (whose growth is 

 usually retarded), much greater doses will, of course, still 

 permit increased growth. 1 



Our chief interest, however, concerns the differential effect 

 of thyroid activity upon bodily proportions, and here Ham- 

 mett himself has made elaborate experiments upon albino 

 rats. Groups of these were thyroidectomized at 23, 30, 50, 

 65, 75 and 100 days respectively, and their organ sizes and 

 weights determined and compared with those of unoperated 

 controls at 150 days. When the increments made by the 

 various organs measured in the operated animals are calculated 

 as percentage of the increments made in the same space of 

 time by the same organs of the controls, some important facts 

 emerge. First, in every case the effect of thyroidectomy in 

 retarding growth (not the absolute effect, but the relative effect, 



1 Thyroidectomy in some animals (anuran tadpoles, axolotls, etc.) 

 is not accompanied by any change in growth-rate. In anuran larvae, 

 excess thyroid is usually accompanied by a decreased growth (incipient 

 metamorphosis). Even here, however, very minute doses accelerate 

 growth during the early part of the pre-metamorphic period. 



