PHARYNGEAL DERIVATIVES OF AMBLYSTOMA 609 



tudinal direction of the body about 200 micra, measured from the 

 caudal side of the hyomandibular pouch and ending with the 

 caudal, lateral projection, just under the anterior wall of the ear. 

 The anlage of the second thymus body is formed from the dorsal 

 angle of this pocket at its caudal extremity, in the same way as the 

 first body was formed from the hyomandibular pouch. This 

 anlage (^u, fig. 8) is not as well defined as the first, so that it is 

 difficult to fix its exact limits. The conical projection is pro- 

 nounced in but a single section, but it is probable that cells of 

 entoderm in front and in back of this contribute to its formation. 

 In addition to the dorsal region, the lateral region of this pouch 

 thickens and extends toward the ventro-lateral wall where the 

 ectoderm is to split to form the first (br.l) branchial cleft. The 

 anlage of the body, as noted above, lies just below the anterior 

 wall of the ear; the glossopharyngeal {IX) ganglion, and the 

 internal carotid artery are medial to it. Near the anlage of the 

 second thymus body are numerous more darkly stained cells, 

 similar to those mentioned in connection with the first. In the 

 ventral region of the body, the structures are not clearly defined, 

 although the pericardium and the truncus arteriosus are 

 recognizable. 



The third pouch (second branchial) is nearly 160 micra long, 

 its caudal extremity lying just posterior to the ear. The anlage 

 of the third thymus body (1^, fig. 9) is formed by a lobe of cells 

 directed dorsally from the distal tip of this pouch, the lateral por- 

 tion of which extends toward the ectoderm of the body wall to 

 form the second {br.2) branchial cleft. This anlage is more 

 clearly defined than that of the second, and occurs in three suc- 

 cessive sections (each 10 micra). The vagus (X) ganglion is just 

 above this anlage, the more darkly stained cells of Driiner inter- 

 vening between the two. The pericardial chamber in the ventral 

 region of the body is here of considerable size, and the heart is 

 flexed. 



The anlagen of the fourth and fifth thymus bodies {t,^, t^, figs., 

 10 and 11), are formed in a similar way as solid outgrowths from 

 the dorsal extremities of the fourth (third branchial) and fifth 



