BRANCHIAL DERIVATIVES IN TURTLES 317 



and in Trionyx of 13-mm. carapace length the ultimobranchial 

 body assumes a structure resembUng that of the thyreoid in the 

 same specimens, but, nevertheless, distinct and readily dis- 

 tinguishable from it by the complete absence of colloid within the 

 vesicles, by the comparatively small number of such vesicles or 

 tubules, as well as by their irregular form, thicker walls, and 

 often ill-defined lumina. A rudimentary right ultmobranchial 

 body is present in all of the later stages described and it under- 

 goes parallel differentiation with that of its much larger fellow. 



In Chrysemys at the time of hatching the ultimobranchial 

 body has assumed an appearance very much like that of the 

 parathjTeoid of the same embryo, namely, a rather lightly 

 staining lymphoid structure in which traces of the earlier ar- 

 rangement and grouping of the cells are clearly recognizable 

 only in a few places. The position of the body remains the same. 

 Deeply imbedded within the left ultimobranchial body lies the 

 parathyreoid IV, which, however, is surrounded by a thin con- 

 nective-tissue capsule of its own. The ultimobranchial body of 

 the right side consists of a small mass of tissue on the medial 

 side of and partly investing the parathyreoid IV; in structural 

 differentiation it is hke its fellow of the opposite side. 



SUMMARY 



1. The development of the branchial derivatives was studied 

 in turtles of the genera Chelydra, Chrysemys, and Trionyx. 



2. The persisting thymus arises from the dorsal portion of the 

 third visceral pouch. In the corresponding portion of the second 

 visceral pouch there is a cellular bud which is interpreted as a 

 rudimentary, transitory thymus. 



3. The ventral portion of the third visceral pouch gives origin 

 to a persisting parathyreoid. 



4. The fourth visceral pouch gives rise to a persisting para- 

 thyreoid, but so far as available material indicates there is 

 no indisputable evidence that a persisting thymus arises from 

 this pouch. A rudimentary thymus which is transitory probably 

 occurs. 



