BRANCHIAL DERIVATIVES IN TURTLES 303 



larger of these two secondary diverticula represents the develop- 

 ing fourth visceral pouch; the smaller one represents an early 

 stage in the differentiation of the diverticulum destined 

 to give rise to the ultimobranchial body. A fifth pouch cannot as 

 yet be positively identified (fig. 1,V, p. 5?), though potentially 

 present. 



At this stage there is on each side, between the pharynx 

 proper and the diverticulum just mentioned, a slender blood 

 vessel which inosculates with both the fourth aortic arch and 

 the aortic root; it ends at the boundary line between the fourth 

 visceral pouch and the ultimobranchial diverticulum. This 

 vessel is shown by later stages to be the fifth aortic arch. 



The next available specimen is an embryo Chrysemys of 6.5 

 mm. (figs. 2, 3, and 13). In this embryo the fourth gill pouch 

 also is perforate. The first three pouches, except for an increase 

 in size, reveal no new features requiring comment. The pharyn- 

 geal outpouching, which in the preceding developmental stages 

 embodied in one the fourth and fifth pouches and the ultimo- 

 branchial diverticulum, is now for the first time distinctly 

 differentiated into its three component parts. Furthermore, the 

 fifth visceral arch has arisen, interposing itself so as to separate 

 the fourth pouch anteriorly from the other two components 

 of the original vesicle. At its pharyngeal end the fourth pouch 

 is broadly continuous with the remaining portion of the vesicle. 

 The latter now consists largely of the ultimobranchial diver- 

 ticulum, the fifth pouch appearing to be merely an anterior, some- 

 what laterally projecting, secondary outgrowth. The complex 

 of diverticula as a whole has been constricted from the pharynx 

 so as to open into it through a short but still relatively wide 

 passage formed by the confluence of the mouth of the fourth 

 pouch with a very short common opening of the fifth pouch 

 and the ultimobranchial diverticulum. The ultimobranchial 

 pocket has a depth equal to about two-thirds the dorsoventral ex- 

 tent of the fourth pouch. It is somewhat elliptical, flattened 

 lateromedially, and its walls are thick. The fifth pouch is small 

 and that of the left side is developmentally more advanced 

 than its fellow. While the fifth pouch, as before remarked, 



