BRANCHIAL DERIVATIVES IN TURTLES 309 



cavity having been obliterated as far back as the pharyngeal 

 stalk. Caudal to this point the pouch has still a conspicuous 

 lumen, but it becomes solid again in the posterior half. In its 

 entirety the third pouch does not exhibit such clear conditions as 

 in Chelydra, and it is uncertain from available material whether or 

 not any particular portion of its wall may be considered as 

 initiating the process of organ formation, such as appears to be 

 the case in Chelydra. In sections through the region of its 

 pharyngeal connection the vesicle has the same crescentic form 

 as in Chelydra and, on one side at least, the lateral wall is notice- 

 ably thicker, but, because of the solidification of the pouch ante- 

 riorly, the original relation or the significance of this thickening 

 cannot be determined. The fourth visceral pouch has a broader 

 connection with the ultimobranchial vesicle and the latter is 

 well developed on both sides of the body, although that of the left 

 is by far the larger. 



In another 10.5-mm. Chelydra a variation in connection with 

 the fourth pouch and the ultimobranchial body should be noted. 

 On the right side a relatively large ultimobranchial vesicle 

 is present. It is spindle-shaped and extends through fifteen 

 sections, having a diameter in its widest part of approximately 

 one and a half times that of the trachea; its walls are thick 

 and the lumen clear-cut. Its anterior end lies just outside 

 the mesenchymal coat of the oesophagus and reaches the level 

 of the parathyreoid III. With this ultimobranchial body the 

 fourth pouch derivative as yet maintains a slender cellular 

 connection (fig. 11), but, instead of being situated at the anterior 

 end of the vesicle, where it is found in most cases, it here lies 

 at the posterior end. How this relation may have been brought 

 about is not evident, but it possibly may be accounted for by 

 assuming that, after the ultimobranchial vesicle had separated 

 from the pharynx, that portion of its neck proximal to the 

 junction of the fourth pouch, in which the limits of the ultimo- 

 branchial vesicle proper are indefinite, continued to develop, 

 while the part distal to the junction suffered regression or had, 

 perhaps, been rudimentary from the outset. On the left side of 

 the body the relations are of the usual kind. The ultimobranchial 



