BRANCHIAL DERIVATIVES IN TURTLES 305 



fifth visceral arch, by its increase in depth, has separated the 

 fourth visceral pouch more widely from the associated fifth 

 pouch and ultimobranchial body. Also a second process of 

 separation, proceedmg simultaneously, is well under way, namely, 

 the pinching off of the complex as a whole from the pharynx by 

 the constriction of the common connecting stalk. 



The fifth visceral pouches in these stages appear to attain 

 their full development as such. In the larger specimens the 

 right pouch is distinctly larger than the left, but in the smaller 

 the two are of about equal size. Contact with the ectoderm is 

 still maintained, but is more restricted than in the preceding 

 stage. On each side of the body a neck-like stalk connects the 

 fifth pouch with the ultimobranchial diverticulum. While, 

 as remarked, the right pouch in the larger specimen of Chelydra 

 is larger than the left, other specimens of this genus as well as 

 of Chrysemys indicate that there is considerable variation in the 

 comparative size of right and left pouches in different embryos. 

 In the 9-mm. Chelydra the long, or dorsoventral axis, of the larger 

 right pouch is about one-fourth that of the fourth \T.sceral pouch. 



The ultimobranchial body, beyond an increase in length 

 and the clearer demarcation noted above, exhibits no important 

 changes. 



A notable feature in connection with the aortic arches at this 

 stage is that the middle segment of the fifth arch, or that which 

 forms the anterior limb of the loop, is exceeded in caliber, although 

 slightly, by the posterior limb or that which represents the s'xth 

 aortic arch. In another embryo Chelydra of 7.5 mm., which in 

 other respects is in a corresponding stage of development, the 

 sixth aortic arch is already much larger than the fifth. In both 

 specimens the pulmonary artery is now present as a branch of the 

 sixth arch immediately above its junction with the fifth. 



In a 9.5-mm. Chelydra, the second visceral pouch has lost 

 its connection with the eetoderm; its dorsal portion shows a 

 thickening of the epithelium which probably represents a transi- 

 tory thymus bud, disappearing with the closure of the pouch. 

 The ectodermal duct is a very much attenuated tube, but has 

 a longitudinal cellular ridge projecting into its lumen from its 

 medial wall (fig. 6). 



