PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 613 



pouch. It is between 35 and 40 micra in diameter and lies just 

 ventro-medial to the levator arcuum branchiarum of the third 

 arch, in the region of the vagus ganglion. 



The anlage of the fifth thymus body (ty, fig. 17), is a small 

 bean-shaped group of cells still in connection with the caudal dis- 

 tal end of the fifth pouch (fourth branchial) and is 120 micra 

 caudad to the fourth body. It is about 20 micra in diameter, and 

 protrudes dorsally into the space between the levator arcuum 

 branchiarum muscle of the fourth arch, and the medially placed 

 dorso-laryngeus muscle. Since the pouch from which this body 

 is derived has been pushed laterally and caudally as noted above, 

 its caudal extremity in section is completely separated from the 

 centr&l pharyngeal tube. In the intervening space, the internal 

 jugular vein passes ventrally, and, medial to the vein, the strands 

 of the dorso-laryngeus muscle extend from the region of the larynx 

 to the lateral wall of the neck region. The small lateralis ramus 

 of the vagus nerve passes backward just above the anlage of the 

 thymus body, the scattered darkly stained cells lie in the space 

 between. 



C. Ainblystoma larvae, 10 to 12 mm. long. Since larvae of these 

 lengths are very similar in developmental details, a single descrip- 

 tion of the thymus bodies in them will answer for all. Growth 

 has been rapid, so that parts which were only outlined as loose 

 cell masses in the earlier stages, are now well defined. This rapid 

 development has its effect upon the thymus bodies as well as 

 upon the other organs and tissues, so that some irregularities 

 occur in their size, shape and general position. 



The first thymus body is now (fig. 18), reduced in size, and 

 although still attached to the pharyngeal wall, it protrudes dor- 

 sally as a finger-shaped stalk, just ventral to the facial ganglion 

 and medial to the postero-lateral surface of the quadrate anlage. 

 Its distal end is now even smaller than its stalk, the whole body 

 being only about 20 micra in thickness, and occurs in but two suc- 

 cessive sections. The mandibular trunk of the internal carotid 

 artery passes to the lateral region of the jaw just cephalad of the 

 thymus body, and the hyo-mandibular ramus of the facial gan- 

 glion passes to the region of the jaw just caudad to it. This 



