METAMORPHOSIS IN FROG LARVAE 527 



the larvae of both thyroid-fed and control cultures were killed, 

 and preserved for microscopic examination of the germ glands. 



The fixing fluids used were potassium bichromate-acetic, and 

 Flemming's solution; both gave excellent results. The sections 

 were cut a thickness of 7.5ju and stained with Heidenhain's iron 

 hematoxylin. A counterstain of congo red was also employed. 



Microscopic examination of the gonads of both thyroid-fed 

 and control larvae revealed nothing of importance. The glands 

 of the various tadpoles of each series varied somewhat in size; 

 there appeared to be no constant size of the glands among the 

 animals as a whole, or of each group. Figure 1, represents a 

 cross section through the gonad of a thyroid-fed animal. When 

 this diagram is compared with that of figure 2 (transverse section 

 through gonad of a control) it is obvious that the marked so- 

 matic changes brought on by the administration of thyroid extract 

 do not extend to the gonads and germ cells. The gonads of the 

 animals of both series are in practically identical stages of de- 

 velopment. The diagrams shown in figure 1 and 2 were taken 

 from representative portions of each gonad. In some of the 

 thyroid-fed animals the glands contained more germ cells than 

 are figured in the cross section of the animals chosen, but this 

 was also true of some of the controls. 



April 28, another series of tadpoles was selected for thyroid- 

 feeding. These larvae were somewhat larger than the animals 

 used in the work just described, averaging 15 mm. in length at 

 the beginning of the experiment. Eighty animals were used, 

 forty for thyroid-feeding, forty for controls. The method of 

 procedure was the same as that reported for series 1. 



May 8, the peculiar body shape characteristic of thyroid fed 

 tadpoles became noticeable; also the limb buds of the hind legs 

 appeaed. The controls showed none of these changes. By 

 May 12, all of the larvae of the thyroid-fed culture were well 

 along in metamorphosis. Such rapid somatic changes, and 

 consequent accelerated metabolism, was accompanied by great 

 mortality among the animals. The following table gives the 

 measurements of the larvae, and also the amount the spiral gut 



