630 FRANCIS M. BALDWIN 



by those of the thyreoid, and are being dispersed into the sur- 

 round ng connective tissue in a manner similar to that described 

 by Miss Piatt in Necturus, and do not form persisting follicles 

 like those which Maurer interpreted as accessory thyreoid in 

 Triton. 



Each half of the thyreoid is now an elongate ovate, mass of 

 epithelial cells (about 150 micra long), lying in the connective 

 tissue, its long axis paralleling the sternohyoideus muscle of 

 either side, the anterior end of the thyreoid appears in the same 

 section with the anterior wall of the ear, and the caudal end 

 stops short of the anterior wall of the pericardial chamber by 60 

 or 70 micra. Although sofnewhat interrupted and irregular in 

 places, the cells of which the halves are composed, are, for the 

 most part solidly packed as in the earlier stages, and they retain 

 to a considerable degree, the yolk-filled character as do the cells 

 of the pharyngeal wall at this stage. The principal blood 

 vessels of the region (the inferior jugular vein, and the exter- 

 nal carotid artery) although faint in outline, are fairly easily 

 recognized; these, and their branches running for the most part 

 through the region lateral, and a little dorsal to the thyreoids of 

 either side. As development proceeds, the cells of each half of 

 the thyreoid pass laterally and dorsally, so that they soon come 

 to lie close to the medial wall of the inferior jugular vein. 



D. Amblystoma larvae, 13 7nm long In the 13 mm. stage, the 

 cells of the thyreoid have changed noticeably in their appearance 

 and arrangement. They no longer form soHd anlagen, but are 

 more scattered throughout the surrounding connective tissue. 

 For the most part they are free from one another, but are arranged 

 in a broken line on either side of the body, paralleling the inferior 

 jugular vein for a distance of about 300 micra. It is difficult to 

 make more than general statements of the arrangement of these 

 cells throughout their extent, since this varies greatly on the two 

 sides of the body. As a rule, the cells have migrated laterally 

 and dorsally, and now are arranged about the vail of the inferior 

 jugular vein, and occupy in places, the spaces between the walls 

 of the jugular and its junction with numerous smaller branches, 

 some even lying in the space between the jugular and the lingual 



