BRANCHIAL DERIVATIVES IN TURTLES 315 



disintegration, while on the right side there is a single much 

 enlarged cyst in which the dorsal and posterior walls alone bear 

 thickenings or proliferating cell masses (fig. 19). The walls of 

 these bladder-like expansions of the fourth pouch derivative at 

 this stage do not, as a rule, possess the clear-cut epithelial ar- 

 rangement of their cells nor the smooth even contour of their 

 inner and outer surfaces which characterized the earlier stages. 

 The cells are notably crowded and jumbled, with here and there 

 dissociated cells intruding into the central cavity. 



But while it appears that in Trionyx the fourth pouch deriva- 

 tive is characterized by the tendency to cyst formation from 

 its early stages and upward, a similar condition, and one which 

 was not foreshadowed in the last-described stage (9.5-mm. 

 c. 1.) of this genus, occurs in the Chelydra embryos of 15-mm. 

 and 16-mm. carapace length (figs. 20, 23). The greatest develop- 

 ment of the vesicular portion is found in the smaller of the two 

 embryos, where it not only exceeds any of the corresponding 

 vesicles in Trionyx, but approaches closely the size of the larger 

 ultimobranchial vesicle in the same embryo. It will be observed 

 from the figures that only a part of the fourth pouch derivative is 

 involved in the cyst, the whole being, as in Trionyx, composed of 

 a glandular and a vesicular part. In the younger embryo the 

 glandular part lies upon the ventrolateral wall of the bladder 

 portion, while in the older specimen it lies upon the ventromedial 

 and the dorsolateral surface, of right and left sides, respectively. 

 At some points the cyst wall has reached a thinness bordering on 

 the breaking-point, where the cells form a single layer and as- 

 sume a mesothelial appearance. In all cases the cyst portion has 

 cellular continuity with the glandular body, although, as in 

 figure 20, the connection may at times be reduced to a very slender 

 stalk. 



The significance of the vesicular portion of the parathyreoid 

 IV is not clear. As to its origin, however, it seems quite certain, 

 from the conditions observed in Trionyx, that it is a part of the 

 original cavity and wall of the fourth visceral pouch. The 

 question will suggest itself whether it may represent a portion of 

 the ultimobranchial vesicle which has separated, along with the 



JOURNAL OF MORPHOLOGY, VOL. 36, NO. 2 



