PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 621 



The third thymus body (fig. 32) is 120 miora caudad to the ear, 

 and lies as an elongate, ovate body of considerable size (240 x 160 

 micra) dorsal and a little medial to the caudal tip of the first 

 branchial arch. As yet its relation to the blood vessels and nerves 

 of the region is little changed; the first efferent branchial artery 

 passes to the dorsal aorta from the gill just below it, and the rami 

 of the glossopharyngeal and vagus nerves pass into the gill region, 

 one in front the other behind the body. The condition of the 

 pharyngeal pouches in this region however show that the clefts 

 are closed to the outside. 



The fourth thymus body (fig. 33) is a smaller oval structure 

 (150 X 100 micra), and is nearly 180 micra caudad to the third, 

 but, because of the reduction of the distal end of the second arch, 

 it lies for the most part caudal to its posterior tip. The second 

 efferent branchial artery, has moved forward, so that now its 

 trunk leaves the medial side of the cartilage well in front of and 

 below the thymus body. The pharyngeal entoderm which 

 formed the gill pouches (third and fourth branchial) is now re- 

 duced to two short blind tubes on either side, which extend 

 caudally and laterally but a short distance from the central 

 cavity of the pharynx. With the closure of the gill slits, and the 

 reduction of the caudal extremity of the arches, the ramus of the 

 vagus to this region has also disappeared. 



Like the fourth, the fifth thymus body (fig. 34) lies well dorsal 

 and for the most part caudal to the cartilage of the fourth arch. 

 It is the largest of the three (360 x 120 micra) and lies about 60 

 micra behind the fourth. 



In the late transforming stage all of the slits are closed, their 

 cori-esponding pharyngeal pouches being recognized as mere stubs 

 of entoderm, and the branchial arches are rearranged to form the 

 hyoid apparatus of the adult. As a result of this extensive shift- 

 ing of parts, the three thymus bodies have been forced somewhat 

 caudad to the ear, the third being nearly 400 micra from it, the 

 fourth and fifth lying closely behind the third. 



The third thymus body is now considerably flattened laterally, 

 having been forced by shifting into the somewhat limited space 

 above the first branchial cartilage, between the dorso-laryngeus 



JOURNAL OF MORPHOLOGY, VOL. 30, NO. 2 



