GROWTH OF AMPHIBIA AFTER THYROIDECTOMY 29 



superior lobe of the hypophysis are irregularly round or columnar 

 in shape with basal nuclei that are smaller than those of the cells 

 in the inferior lobe. The anterior lobe consists of cells with 

 very little cytoplasm and with large nuclei about the size of 

 those of the inferior lobe or slightly larger. 



The hypophysis may be the source of the stimulus which 

 causes the thyroidless larvae (with enlarged hypophysis) to grow 

 more rapidly than the controls, although the hypertrophy of this 

 gland was not noticeable until after the time at which the differ- 

 ence in the rate of growth began. However, the variability of 

 size of the hypophysis is considerable and it is also true that the 

 secretory activity of a given organ is not necessarily exactly 

 proportional to its size. An organ with a rapid secretory rate 

 would produce more secretion than a somewhat larger organ 

 with a slow rate. It is also true that the chemical nature of the 

 secretion of the hypophysis may be changed by removal of the 

 thyroid. 



Thyjnus. The thymus in frog larvae is paired, one of the 

 pair located on each side laterally between the eye and ear and 

 dorsal to, but near the gills. In the change of form through 

 which the animal goes at metamorphosis, the depressor maxillae 

 grows posteriorly between the eye and the thymus and pushes 

 the latter posteriorly and ventral to the junction of the jugal, 

 pterygoid, and tympanicum. 



The thjrmus reacts to thyroidectomy in much the same way 

 as all other organs except the hypophysis. It is not affected at 

 all up to the time of metamorphosis or at least no effect can be 

 determined either in size or structure. Our figures 46 to 49 

 represent a few of the shapes this organ assumes, and others 

 are shown by Rogers (p. 605). This variation is even greater 

 than that of the hypophysis, and the objection made to Rogers' 

 method of calculating the volume of the latter gland applies 

 here. He states that the errors made in the calculation for 

 different groups counterbalance each other, but, as he shows 

 on page 605, there were relatively few glands measured, and the 

 typical shape of the gland in the different groups is not the same. 

 Some of the thymi are nearly cylindrical, some (especially after 



