612 FRANCIS M. BALDWIN 



from which they have separated. This group is located well to 

 the side of the body, in the region of the glossopharyngeal 

 ganglion and lies equidistant between the medial surface of the 

 levator arcuum branchiarum muscle of the first gill arch, and the 

 internal jugular vein. It is just in front of the point where the 

 pretrematic ramus of the glossopharyngeal nerve enters the gill 

 arch. The body is almost spherical in shape, and is about 30 

 micra in diameter. The darkly stained cells are now less in evi- 

 dence; only one or two appear in each section. 



The caudal region of the third pouch (second branchial) lies 

 about 90 micra caudad to the second. The anlage of the third 

 thymus body (^„i, fig. 15) is not yet completely separated from 

 the dorsal cells of the pouch, having a very narrow connecting 

 stalk of cells. This body is very small, between 15 to 20 micra in 

 diameter. It lies just posterior to the second efferent branchial 

 artery on its course from the arch to the radix aorta. Caudad to 

 the gland, the first branchial nerve of the vagus passes into the 

 arch. The jugular vein lies in the same relative position as 

 before, but the internal carotid artery does not extend as far 

 back as this region. The darkly stained cells in the region of 

 this thymus body have disappeared. Maximow found the third 

 thymus body in Siredon, 9.5 mm. long, free in the mesenchyme, 

 dorsal to the dorsal wall of the pharynx, and in close connection 

 with the vagus ganglion, and medial to the large muscle ('wahr- 

 scheinlich den M. levatores arcuum branchialium'), and some- 

 what larger (40 micra in diameter) than the one I have described 

 in Amblystoma of this stage. 



The caudal extremities of the fourth and fifth pouches (third 

 and fourth branchial) are more dorsal in position than any of the 

 others, so that the points from which the fourth and fifth thymus 

 bodies develop, lie above the level of the notochord, and close 

 under the dorso-lateral wall of the larval body. 



The caudal distal extremity of the fourth pouch, from which the 

 fourth thymus body is developed (^iv, fig. 16), lies 60 micra caudad 

 to the third thymus body. Its anlage is an elongated, ovoid 

 group of cells still in connection with the dorsal portion of the 



