22 E. R. HOSKINS AND M. M. HOSKINS 



hiding it and pressing upon it so that it becomes more flattened 

 and appears to become smaller. 



Allen ('18) noted this early lack of differentiation of the brain 

 of the thyroidless larvae, but did not observe the changes which 

 occur later. It is to be noted that this differentiation of the 

 brain is independent of the changes that occur normally during 

 metamorphosis. The reason for this slow growth of the brain 

 of the thyroidless larva is not evident. In control and thyroid- 

 less larvae of the same size the brain in the latter is usually, 

 though not always distinctly smaller, but it is always less highly 

 differentiated in the thyroidless than in the control larva during 

 and for some time after metamorphosis of the control. The 

 brain during metamorphosis, loses very little, if any, in size and 

 hence changes greatly in relative size; so that, relatively, a nor- 

 mal larva has a much smaller brain than a frog, and a thyroidless 

 larva a still smaller one. 



Eyeballs. The eyeballs of the metamorphosing controls are 

 of about the same size as those of the corresponding thyroidless 

 larvae. During metamorphosis the eyeballs remain about the 

 same size as before or perhaps gain shghtly, so that a frog has 

 relatively larger eyes than a control or thyroidless larva. In 

 the latter, however, the eyeballs continue to grow and this differ- 

 ence is somewhat decreased. During metamorphosis the head 

 narrows and the eyeballs approach each other medially, but 

 as the brain loses but little if any in width the optic nerves must 

 be obliged to shorten (compare figs. 13 and 17). 



Heart. The heart (figs. 28 to 31) has about the same propor- 

 tions in the control and thyroidless larvae, but varies in size 

 as seen in autopsies on account of varying amounts of enclosed 

 blood. During metamorphosis in the normal animal the heart 

 loses a little in size, but less than the body as a whole. In the 

 older thyroidless larvae, the heart increases in size while the 

 body is growing. 



Liver. At the time of metamorphosis the liver is very irregular 

 in outline (figs. 32 and 33). A coil of the intestine half embedded 

 in it increases its irregularity. The groove made by the intestine 

 is indicated in the figures. The liver consists of three principal 



