18 E. R. HOSKINS AND M. M. HOSKINS 



practically cease before the frog skeleton is laid down and before 

 the other parts of the larval body have ceased growing. It is 

 probable, as in mammals, that thyroid deficiency interferes with 

 calcium metabolism and thus causes the skeletal abnormalities 

 noted here. The failure of the legs to grow beyond mere buds 

 may be due to the failure of the skeletal part to push out into 

 these buds. 



The beginnings of metamorphosis are to be observed in the 

 skeleton, which changes to accommodate the body of the animal 

 •to its future terrestrial existence. Metamorphosis consists of 

 a series of changes which occur in sequence, and when the first 

 part of the process is prevented (i.e., skeletal change) through 

 faulty metabolism (probably calcium) other steps in the sequence 

 are prevented. It is to be noted that the lungs develop and 

 function before metamorphosis begins, and have nothing to do 

 with this phenomenon. As soon as normal larvae are well started 

 in metamorphosis they may be removed from the water, and 

 the process will continue to its completion. 



It is of course possible that thyroid secretion is directly neces- 

 sary for the atrophy of tissues which occurs in metamorphosis, ' 

 but the experiment in forced metamorphosis described above 

 showed that a certain amount of atrophy will take place in a 

 thyroidless larva kept in a moist chamber. 



The hypertrophy of the hypophysis which occurs after thy- 

 roidectomy may have something to do with the failure of meta- 

 morphosis, but it is more likely "that the condition of this gland 

 is responsible for the rapid growth rate of thyroidless larvae. 



There is no apparent reason for the fact that the size at which 

 frog larvae go into metamorphosis is not constant. Animals 

 from the same egg mass kept in the same aquarium and hence 

 under similar food and temperature conditions may vary con- 

 siderably in the size attained before metamorphosis and in the 

 time required for it. As noted above, the 1917 (slowly growing) 

 larvae averaged larger than the 1918 (rapidly growing) larvae, 

 but the food conditions are not the only determining factor, 

 because the largest 1918 larvae were larger than the smaller 1917 

 larvae. It may be that about the time the larvae reach their 



