HETEROGONY AND HORMONES 181 



the few days between its administration and metamorphosis, 

 causes a progressive geometric increase in the number of 

 dividing cells. This would imply a progressive increase in 

 growth-ratio during the period. Further researches on this 

 interesting point are needed. 



In any case, the legs of Anura are of interest to us in several 

 respects. First of all, they emphasize once more the import- 

 ance of tissue-specificity. While their growth responds very 

 sensitively to thyroid, the larval Urodele limb is wholly in- 

 sensitive to the same hormone. Next, they show us that 

 even the threshold of reactivity to one and the same hormone 

 may vary from tissue to tissue. Anuran limb-buds appear to 

 be sensitive to any dose of thyroid, starting from zero. But 

 the equally specific degenerative response of the tail-tissues 

 does not begin until a considerable concentration of thyroid- 

 hormone is reached in the blood. Thirdly, they show us a 

 clear-cut case of the growth-coefficient of an organ varying 

 within wide limits with the dosage of a hormone. Even the 

 normal growth of the Anuran tadpole's limbs is slightly hetero- 

 genic, as is shown by extirpating the thyroid in the embryo, 

 when growth is much lower, and apparently isogonic (Allen). 



Fourthly, they show in diagrammatic form the interrelation 

 of the factors of growth-coefficient and available time as regards 

 the problem of relative size of organ. Normally, the slight 

 leakage of thyroid hormone into the blood during larval life 

 produces a mild limb-heterogony. By the time the meta- 

 morphic crisis occurs (apparently due to a specific change in 

 the pituitary which causes the thyroid to liberate most or all 

 of its secretion suddenly into the blood), the hind-legs, originally 

 mere buds, have had time to increase in relative size until 

 longer than the trunk, although the trunk itself has increased 

 perhaps two- or three-fold in linear size. 



If, on the other hand, a moderately strong dose of thyroid 

 be administered to small or small-medium tadpoles, with limb- 

 buds hemispherical or conical but not yet fully differentiated, 

 then although the growth-ratio of the limb-bud is raised far 

 above normal, the accelerated growth can only operate for a 

 few days before metamorphosis supervenes, with the result 

 that the transformed froglet has relatively very small limbs. 

 It would be of great interest to see whether these proportions 

 were later regulated to or towards the normal, but the experi- 

 ment has not so far been tried. As suggested in Chapter II, 

 this factor of the amount of time available for heterogonic 



