PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 637 



upon the wall of the inferior jugular vein. The gland appears in 

 the same section with the anterior wall of the ear, but the anterior 

 wall of the pericardium is about 250 micra caudad to its posterior 

 end. This relationship is interesting, since in the next stage 

 there is a difference in the relative position of the heart and the 

 gland. 



When transformation has proceeded so far that the arrange- 

 ment of the hypoglossal muscles, branchial arches, blood vessels 

 and other parts is practically that of the adult, only the hyoid and 

 the first and second branchial arches remain, the latter (second 

 arch) is much reduced in size and length; the gill-slits are com- 

 pletely closed, only mere rudiments of pharyngeal entoderm 

 marking their former position. Simultaneously with this rear- 

 rangement, the thyreoid gland of either side has been converted 

 into a compact, egg-shaped mass of follicles, and has migrated 

 laterally and a little caudally, so that it now lies lateral to, and a 

 little in front of the pericardial chamber, in the space just back of 

 the caudal tip of the second branchial cartilage, between the ad- 

 ductor muscle of the first branchial cartilage, and the large medial 

 sternohyoideus muscle. The position of the gland relative to the 

 cartilages and blood vessels is shown in figure 46. 



The follicles of the gland vary greatly in size and shape, and in 

 the color of the colloidal contents, some follicles being light gray, 

 others a brownish or yellowish hue. The differences in the num- 

 ber and size of follicles and the variety in their histological appear- 

 ance, when compared to those of the later, larval stages, and to 

 those of the adult, would indicate that the gland itself is under- 

 going a metamorphosis of some sort. Although it is not the pur- 

 pose to enter here a discussion of the histology of the gland, it is 

 very evident that the larger follicles are g'ving rise to smaller 

 ones by budding, and that this process of follicular formation, 

 begun in the late transforming stage, continues in the adult. 

 Different opinions have been expressed as to the method of forma- 

 tion of follicles throughout the vertebrates (Baber, '76; Anderson, 

 '94; Forsyth, '08; and Gudernatsch, '11). 



L. Adult Amblystoma. The relations of the thyreoid gland 

 to the structures in the neck region of several adult urodeles, has 



JOaRNAL OF MORPHOLOGY, VOL. 30, NO. 2 



