308 CHARLES EUGENE JOHNSON 



The fourth pouch has by this time also developed into an 

 almost entirely solid body, club-shaped in form, the tapering end 

 directed forward and slightly marked off from the posterior part, 

 as if it represented a rudimentary thymus. The pouch extends 

 through eleven sections (175m), the three middle sections alone 

 containing evidence of the former cavity. The caudal end is in 

 close proximity to the ultimobranchial body from which it ap- 

 parently has just become separated. The ultimobranchial vesicle 

 of the left side (fig. 9) shows a very considerable increase in size 

 and is expanded so as to be nearly circular in cross-section, but 

 it has the same smooth-walled appearance as in preceding stages. 

 In greater part the wall shows three or four tiers of nuclei, but 

 in some places there is only one. Its anterior extremity bears 

 a small cellular peg which evidently fixes the point of separation of 

 the fourth visceral pouch. 



The conditions of the right side in this embryo deserve notice 

 in that there apparently is complete absence of the ultimobranchial 

 body; it is the only instance in my series where this occurs. A 

 slender cellular stalk, similar to that of the left side, extends from 

 the pharynx to the fourth pouch, to which it furnishes a short 

 pedicle, and then ends only three sections beyond this point, 

 without discernible evidence of an ultimobranchial vesicle. 

 However, the Hmits between what constitutes the ultimobranchial 

 vesicle proper and the part which represents more or less of the 

 drawnout portion of the pharyngeal wall cannot in any case be 

 exactly determined, and therefore, in view of the conditions found 

 in subsequent stages relative to the point of connection between 

 the fourth pouch and the ultimobranchial vesicle, it is still possible 

 that the latter is potentially present, though in a very rudimen- 

 tary form, in the distal portion of the entodermal stalk. 



The embryo Chrysemys, in corresponding stage of develop- 

 ment, shows a condition of the branchial derivatives similar to 

 that of Chelydra, with minor variations. The second \dsceral 

 pouches have identical tube-like extensions (ectodermal ducts), 

 but these are without cellular buds or areas of prohferation. The 

 third pouch is somewhat more advanced. Its anterior portion is 

 a solid mass of more or less lobular appearance, the original 



