14 E. R. HOSKINS AND M. M. HOSKINS 



increase very slightly in length, the head and back become very 

 flat on account of skeletal changes, and the eyes are relatively 

 more lateral in position than in normal larvae. Other changes 

 noticeable are a relative increase in the length of the tail and a 

 blunting of the anterior end of the head. Allen ('18) states 

 that the head is relatively long, but this is not the case in our 

 specimens. In his comparison Allen ('18, fig. 3) failed to super- 

 impose the centers of the heads of the larvae. At the end of 

 the period of metamorphosis the thyroidless larvae are from 

 three to five times as large in volume as the frogs of the same 

 age and some become ultimately more than six times as large 

 as the average frog after metamorphosis and nearly three times 

 as large as the average control larva before metamorphosis 

 (table 2). 



Table 2 shows the variation in the volume (and hence weight) 

 of amphibians as compared on a basis of length both in the living 

 and fixed condition. These animals were fixed in Bouin's fluid 

 and preserved in 70 per cent alcohol. The data here were se- 

 lected from a very large number of such measurements and 

 published in order to demonstrate a source of serious error in 

 attempts to compare relative volumes of organs on a basis either 

 of length or volume of the animal, as well as to show the relative 

 sizes of larvae and frogs. The volume of fixed specimens varies 

 greatly on account of variations in the amount of shrinkage 

 during fixation, and similar variations in the size of fixed organs 

 must also occur. 



The volume (or weight) of a living larva of a given length 

 is only fairly constant and varies not only with the amount of 

 foreign substances within the intestine, but also on account of 

 variations in shape produced by changes in the tail-body re- 

 lationship. The intestinal contents cannot be removed without 

 destroying the gut, and this would prevent the measurement of 

 the volume (or weight) of the entire animal. In addition to 

 loss of water, a considerable portion of the natural decrease in 

 size suffered during transformation from the tadpole to the frog 

 is due to discharge of intestinal contents, and the exact amount 

 of this is difficult to determine. This loss of fecal matter in- 



