BRANCHIAL DERIVATIVES IN TURTLES 307 



Eegarding the fifth pouch, the gap in my series between the 

 present stage and the preceding is too great to indicate what has 

 taken place in the meantime. In the present specimen there is a 

 small mass of cells lying between the fourth pouch and the ulti- 

 mobranchial vesicle, just behind the point of connection between 

 these two; the mass has the appearance of undergoing degenera- 

 tion, and it is possible that it represents the remnants of the fifth 

 pouch. 



The ultimobranchial body of the left side is typical for the stage 

 under consideration — an elongate tube lying lateral to and 

 parallel with the trachea. It is largest in its middle portion and 

 tapers more or less towards the ends. The walls are of uniform 

 thickness and the enclosed ca\'ity is sharply defined. Proximally, 

 the vesicle narrows rapidly in approaching its connection with the 

 fourth pouch, and from this point on it becomes merely an at- 

 tenuated pedicle connecting the two vesicles as a unit with the 

 pharynx. Close to the entodermal wall this stalk is about to be 

 constricted off, but within it a pinhole cavity is visible. 



The next step is based upon a 10.5-mm. Chelydra, a 6-mm. 

 Chrysemys, and a 9-mm. Trionyx. In Chelydra the third 

 visceral pouch has been transformed into an elongate, compact 

 mass. The anterior two-thirds is considerably larger than the 

 caudal third and it contains a vestige of the original ca\aty, 

 around which the innermost cells retain in slight degree their 

 epithehal character. Anteriorly, and to a less extent in other 

 parts, the mass sends out a number of solid mounds of cells, which 

 give it a somewhat lobular appearance. The smaller caudal 

 mass is a continuation of the medial part only of the anterior mass. 

 It is cylindrical and, like the anterior part, contains a trace of 

 the earher lumen. In brief, the conditions just described simply 

 mean that the third pouch at this stage shows definite differentia- 

 tion into an anterior thymus body and a posterior parathyreoid 

 body, representing, respectively, dorsal and ventral portions of 

 the original visceral pouch. In the specimens of Chrysemys 

 and of Trionyx the third pouch is developmentally slightly more 

 advanced, but otherwise it presents conditions similar to those 

 just described. 



