620 FEANCIS M. BALDWIN 



shape but larger (118 x 95); and the fifth (shghtly flattened 

 medially) is 135 x 105x80 micra. In histological appearance how- 

 ever, two noticeable changes have taken place. The epithelial 

 cells are now much smaller and more numerous and, except for 

 the central area, are scattered loosely throughout the mass (fig. 

 30) . The outer margin of the mass is now surrounded by a loose 

 connective tissue envelope, strands of which pass between the 

 small epithelial cells. 



Aside from considerable increase in size no great difference is 

 noted in the bodies in the 39 to 40 mm. larvae. The three bodies 

 are oval; the third being 200 x 155 micra, and the fourth and fifth 

 measuring 152 x 122 micra and 175 x 135 micra respectively. 



In the last stage studied before the larva undergoes its trans- 

 formation (45 mm. long), all of the thymus bodies have increased 

 markedly in length, but no great change in diameter has taken 

 place. The third is now 220 x 164 micra, and lies about 125 micra 

 caudad to the ear; the fourth is 180 x 240 micra and the fifth is 

 285 x 350 micra. Because of this increase in length, the caudal 

 end of the third, and the cephalic end of the fourth are close to- 

 gether (separated only about 40 micra). These two lie caudad 

 and a little lateral to the levator arcuum branchiarum muscle of 

 the second arch. The fifth is about 200 micra caudad to the 

 other two, it is somewhat elongate and its cephalic end is inserted 

 into the space between the distal tips of the third and fourth 

 branchial cartilages, and the medially placed dorso-laryngeus 

 muscle, its caudal end tapering to a point close to the lateralis 

 ramus. 



G. Transforming Amblystomae. The caudal extremity of the 

 gill region in the early transforming larvae is undergoing most 

 modification, so that the fourth and fifth (third and fourth bran- 

 chial) clefts are closed, the pharyngeal entoderm of these per- 

 sisting as mere rudiments. This readjustm< " the caudal 

 region of the gills has its effect upon the three thymus bodies, 

 especially the fourth and fifth, which, as earlier pointed out, were 

 intimately associated with these pouches in their genesis. The 

 following description is based upon a wax model of the three bodies 

 in this stage. 



