AMPHIBIAN METAMORPHOSIS 609 



found to be reflected in the relative sizes of their thyroids, 

 those of the Alpine tadpoles being the largest. Thus we may 

 say that the inherited developmental rate of the thyroid appears 

 hereto be adaptively connected with environment, a larger thy- 

 roid being found where it is advantageous to counteract the 

 retarding effects of low temperature. That this view is in all 

 probability correct is shown by experiments on the direct effect 

 of temperature on development and metamorphosis. If a single 

 egg-batch is divided into several portions, and these kept at 

 various temperatures, it is found that the larvae at low tem- 

 perature grow to a considerably larger size before metamor- 

 phosing. Witschi in a recent paper (33) has shown that this runs 

 parallel with a differential effect of temperature upon the 

 two antagonistic sides of the metabolic cycle. Tadpoles kept 

 at low temperatures are plump and well nourished, with large 

 livers well stocked with glycogen ; those kept at high tempera- 

 tures are thinner, with few reserves. On the other hand, 

 Adler (2) has shown in similar experiments that low temperature 

 induces a large, high temperature a small, larval thyroid, 

 this being a somatic effect as opposed to the genetic difference 

 previously mentioned. Thus the differential effect of tem- 

 perature upon metabolism is exerted against the thyroid, so 

 to speak. In spite of the thyroid's well-known effect in in- 

 creasing basal metabolism by increasing catabolic processes, 

 the large-thyroided tadpoles at the low temperatures have 

 anabolic processes in excess. This can only mean that the 

 thyroid acts as a regulator of metabolism in these cold-blooded 

 larvae as well as in adult mammals, and, just as in mammals, 

 undergoes functional hypertrophy when an extra demand is 

 made upon it, and shuts down when not needed. This possibly 

 has a bearing on the curious fact that while basal meta- 

 bolism of women, even when corrected for size, is lower than 

 that of men (24), their thyroids are slightly larger (19). This 

 is perhaps a compensatory effect, which, however (as in the 

 low-temperature tadpoles), is not fully adequate to make up 

 for the lower metabolism of the female. 



The low-temperature tadpoles show clearly that it is not 

 the absolute size of the thyroid which determines metamor- 

 phosis, since in them metamorphosis does not occur until the 

 animal has grown to a size above that normally attained by 

 tadpoles, although the thyroid is above the average in size 

 (and apparently in activity, since the histological picture is 

 not abnormal). 



The importance of the tissues in this partnership is well 

 shown by the histological work of Champy (6), who, in tadpoles 

 made to transform by a single meal of thyroid, investigated 

 the " mitotic coefficient," or number of mitoses per thousand 



