304 CHARLES EUGENE JOHNSON 



appears to be a secondary diverticulum from the ultimo- 

 branchial pocket, this is evidently the result of the early 

 differentiation of the ultimobranchial vesicle, involving as it 

 does a relatively large area on the primary pharyngeal outpouch- 

 ing, one in which the diminutive fifth pouch is unable, as it 

 were, to express itself until at a somewhat later stage. The short 

 common passage or stalk previously referred to, by which the 

 fifth pouch and the ultimobranchial diverticulum join the fourth 

 pouch in opening into the pharyngeal ca\dty, evidently represents 

 originally a portion of the pharyngeal wall proper, and the 

 relations existing are consequently of secondary nature. The 

 rudimentary fifth pouch had been carried bodily out from the 

 pharynx by the ultimobranchial evagination. 



In the fifth visceral arch at this stage there is a complete 

 aortic arch, which, about one-fifth of its distance from the 

 dorsal aorta, gives off a more slender posterior branch, the sixth 

 aortic arch; this, after making a loop about the fifth visceral 

 pouch, rejoins the fifth arch. The sixth aortic arch of the right 

 side is incomplete ventrally. The sixth aortic arch hes in the 

 angle formed by the ultimobranchial diverticulum and the fifth 

 pouch, the former being wholly medial to the vessel. 



The fifth \dsceral pouch is in intimate contact with the ecto- 

 derm along its lateral edge. The ultimobranchial outgrowth 

 nowhere touches the outer germ layer; its basal or proximal end 

 is opposite the origin of the trachea, and the distal end is directed 

 ventrally, parallel to the long axis of the fourth pouch. 



In an embryo Chelydra of 7.5 mm. and one of 9 mm., the second 

 and third pouches are still perforate, and in the latter specimen 

 the fourth also is open. The pouches are much flattened antero- 

 posteriorly and their dorsoventral axes have increased consid- 

 erably in length. Because of the greatly narrowed ectodermal 

 and entodermal connections, the dorsal and ventral portions of 

 the second and third pouches appear in the sections as closed 

 vesicles and the posterior wall of the dorsal extensions of these 

 two pouches is now much thicker than the anterior wall. A 

 more pronounced advance, however, is apparent in connection 

 with the posterior complex of diverticula (figs. 4 and 5) where the 



