PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 623 



structure of considerable size (3 to 6 mm. long, and 0.5 to 1.3 

 mm. broad), lying in the loose connective tissue just dorsal to 

 the caudal region of the first branchial cartilage, and closely 

 pressed against the deeper lying dorso-laryngeus muscle (d.J.). 

 (In some cases the surrounding connective tissue is deeply pig- 

 mented in the region of the gland so that its light color is masked, 

 in others this pigmented area is some distance below, enveloping 

 then the carotid gland and epithelial bodies, described below). 



The three lobes, one behind the other, are usually in a line, a 

 little oblique to the longitudinal axis of the body. The anterior 

 lobe (probably the third thymus body of the larva) is a flattened 

 bean-shaped body (between 1 and 2 mm. long, and 0.3 mm. 

 broad) lying in the angle between the mandibular-pectoralis 

 (m.p.) and the cephalo-dorsalis (digastric) (di.) muscles, its 

 cephalic tip being partly covered by the caudal margin of the 

 latter. The second lobe (probably the fourth larval body) is 

 almost spherical (about 1.5 mm. in diameter), and lies a little 

 dorsal to, but close behind the first. The posterior lobe (prob- 

 ably the fifth larval body) is a flattened ovoid body and is the 

 largest of the three. 



The large blood vessels (figs. 36 and 40) in this region — the 

 internal jugular vein, the internal carotid artery and the aortic 

 arches— ^are exposed by removing the digastric, mandibular, and 

 pectoralis muscles, and reflecting the cartilage and muscles of the 

 hyoid apparatus. The connective tissue in which the gland is 

 imbedded is well supplied with blood. A small branch from the 

 second aortic arch and one from the external carotid artery enter 

 the region. (For the details of the vascularization of the thymus 

 gland in adult Axolotl and a discussion of its histology, see Dustin 

 '11). 



The region of the gland is innervated by several rami of the 

 glossopharyngeal {IX) and vagus (X) nerves. These emerge from 

 the region just caudad to the ear (can be seen after reflecting the 

 muscles in this region), the gland itself and its surrounding con- 

 nective tissue being innervated by a small twig from the ramus 

 cutaneus (IX) as Fischer ('64) long ago pointed out in Siredon. 



