312 CHARLES EUGENE JOHNSON 



An embryo Chrysemys and one of Trionyx of 8-mm. and 

 9-mm. carapace length, respectively, show a general develop- 

 mental stage corresponding to the preceding embryo Chelydra. 

 In Chrysemys the derivaties of the third visceral pouch together 

 form a more or less rounded three-lobed mass, partly encircling 

 the carotid artery from the dorsal side (on the left), or from the 

 medial side (on the right). Two larger anterior lobes constitute 

 the thymus, while the third lobe, smaller and situated posteriorly, 

 is the parathyreoid body, the two still having cellular continuity. 

 The parathyreoid here is lateral to the thymus instead of caudal, 

 as in Chelydra, possibly due to a growth or shifting caudad of the 

 thymus. The fourth pouch derivative and the ultimobranchial 

 body have the same relative positions as in Chelydra. The latter 

 body here likewise attains its maximal size as a vesicle, but is 

 relatively and absolutely much smaller and has the general form 

 of a cylindrical tube. The vesicle of the opposite side is 

 rudimentary. 



In Trionyx the thymus and its associated parathyreoid III 

 have the same tandem arrangement as in Chelydra. The fourth 

 pouch derivative may lie against the medial side of the systemic 

 arch, or between this vessel and the pulmonary arch, opposite 

 the bifurcation of the trachea. On both sides of the body the 

 walls of this derivative are somewhat thickened, but maintain 

 an even epithelial arrangement about a relatively large central 

 cavity, as in the earlier 9-mm. stage. The right ultimobranchial 

 vesicle is very rudimentary; the left one is even smaller than that 

 of Chrysemys, and is profusely covered with cellular excrescences, 

 especially in its posterior portion. 



LATER DIFFERENTIATION 



In the well-advanced embryos just described the various 

 branchial derivatives have been identifiable, largely or entirely 

 by their respective histories and place relations. Actual struc- 

 tural differences in the thymus, the parathyreoids, and the 

 fourth pouch derivatives are, even in the oldest of these embryos, 

 wanting or at least uncertain in the sections. The form of the 

 dominant ultimobranchial vesicle renders this organ unmistak- 



