PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 611 



stage with the ectoderm. In addition, at no time in the subse- 

 quent stages do these darker cells, so far as I am able to judge, 

 unite either directly with the thymus bodies or with the nerve 

 ganglia, and therefore, I find no good reason for regarding them 

 as other than mesodermal cells of the region. 



B. Amblystoma larvae, 9.5 7nm. long. In 9.5 mm. larvae, the 

 mouth is not yet open, but the pharyngeal pouches have extended 

 laterally so that the entoderm of their distal walls has fused with 

 the cells of the ectoderm of the body, although the clefts are still 

 closed. The pharyngeal entoderm is now lighter in color, due to 

 its less amount of yolk content, so that the morphological relation- 

 ships of parts are more easily followed. 



The anlage of the first thymus body (fig. 12), has not greatly 

 changed in its relative position. The first pouch (hyomandibular) 

 has extended laterally and at the same time a little cau dad, the 

 thymus body being carried a corresponding distance in the same 

 direction. It is now a somewhat spherical knobule of entodermal 

 cells (about 30 micra in diameter), still connected with the dorsal 

 wall of the pouch, extending dorsally just lateral to the facial 

 {VII) ganglion. Of three successive sections which pass through 

 it, only in the second is it connected with the pouch by a thin 

 stalk of cells. The internal jugular vein is just above, while the 

 internal carotid artery is medial to it. The darkly staining cells 

 of Driiner form a somewhat loose mass between it and the facial 

 nerve. In front of the thymus, the internal carotid artery sends 

 the mandibularis branch to the region of the jaw, while caudad to 

 it, the facial ganglion gives off two rami; one (ramus palatinus) 

 courses medially, the other, a large single trunk (which breaks up 

 into the ramus hyomandibularis, the ramus alveolaris and buc- 

 calis), passes lateral and enters the jaw region (Herrick; Strong; 

 Coghill). 



The second pouch (first branchial) is much like the first; its 

 caudal portion meets the ectoderm of the side of the body, below 

 the posterior wall of the ear, where the gill cleft is to open. The 

 anlage of the second thymus body in this stage is a group of cells, 

 (fig. 14, ^ij) similar in color to those of the dorsal wall of the pouch 



