BRANCHIAL DERIVATIVES IN TURTLES 311 



walls. These growth processes have been most active in the 

 anterior portion of the vesicle, but are present in varying degree 

 throughout its length. In the midst of the proliferating mass, 

 however, the walls are sufficiently intact to show what had been 

 the general form and size of the vesicle at the height of its develop- 

 ment, and in these respects it bears closer resemblance to Chry- 

 semys than to Chelydra, as it evidently attains neither the large 

 size nor the thin-walled condition of the latter. The right 

 ultimobranchial vesicle is a thin-walled tubular structure whose 

 epithelium consists of one or two layers of flattened, loosely ar- 

 ranged cells, evidently in process of retrogression. The fourth 

 pouch derivatives are both characterized by a highly vesicular 

 condition, quite in contrast to the usual solid cellular mass in 

 corresponding stages of the other two genera, but a tendency 

 toward which was seen in the 6-mm. Chrysemys. The walls 

 of these vesicles retain, in part, their early sharply defined epithe- 

 hal form, in part contain foldings and thickenings due to cell 

 proliferation. The tendency of the fourth pouch derivative in 

 Trionyx to assume a vesicular form occurs in later stages and 

 appears to be a distinctive feature of this genus. 



Figure 14 represents a wax-plate reconstruction of the bran- 

 chial derivatives of the left side of an embryo Chelydra of 9.5- 

 mm. carapace length. The thymus and the parathyreoid III 

 maintain their earlier linear arrangement and partly encircle the 

 carotid artery. The fourth pouch derivative, still attached to 

 the ultimobranchial vesicle, lies medial to and occupies the inter- 

 val between the systemic and the pulmonary arch (the latter 

 omitted in the model). The ultimobranchial vesicle has attained 

 relatively enormous proportions, the maximal in my series, 

 having a diameter approximately one-half that of the oesophagus. 

 Only on its anterior and anterodorsal surfaces do the sections 

 reveal cellular outgrowths and extensions from the otherwise 

 smooth wall of the vesicle. Its fellow of the opposite side is 

 relatively insignificant and the fourth pouch of this side is also 

 much inferior in size and is furthermore completely detached from 

 the ultimobranchial body. 



