310 CHARLES EUGENE JOHNSON 



vesicle extends through twenty-five sections; while its walls 

 are thicker than in the preceding embryo — indicative, as a rule, of 

 an earlier stage — it shows a more advanced condition in that they 

 bear a number of secondary evaginations of various sizes as well 

 as numerous solid protrusions or sprouts. Both kinds are es- 

 pecially large and conspicuous about the anterior end of the 

 vesicle, while minor ones occur somewhat distal to its middle 

 section. 



In a 6-mm. Chrysemys, representing approximately the same 

 developmental stage as the foregoing embryo, a further variation 

 with respect to the fourth visceral pouch and the ultimobranchial 

 vesicle occurs. The left fourth pouch has been converted into 

 a compact cellular mass with even surface contour and without 

 trace of lumen, and is attached in the usual manner by a solid 

 stalk near the anterior end of the ultimobranchial vesicle. The 

 last named, except for its smaller size, is similar to that of 

 the 10.5-mm. Chelydra. The right fourth pouch derivative is 

 much longer than the left (330/^ as against 240) and its middle 

 portion is expanded into a vesicle of nearly the same diameter as 

 the ultimobranchial vesicle itself (fig. 18), into which it opens by 

 a passage extending through five sections; and the ultimo- 

 branchial vesicle is unusually large for this side, being somewhat 

 more than half the length and width of the left one. 



A 9-mm. Trionyx is the youngest specimen of this genus n my 

 possession. In general development it agrees well with the 

 preceding specimen of Chrysemys. The derivatives of the third 

 visceral pouch reveal no noteworthy differences from those of 

 corresponding stages of Chrysemys or Chelydra; but the fourth 

 pouch derivative and the ultimobranchial vesicle show distinct 

 variations from the conditions in those genera. On the left side 

 the two bodies in question have the usual position relative to 

 each other and have a cellular connection, but in form they are 

 of somewhat different type. The ultimobranchial vesicle is 

 much more advanced in development than that of either Chry- 

 semys or Chelydra of corresponding age in that a large portion of 

 it has already been transformed into solid cord-like cell-clusters, 

 while elsewhere it bears spherical, hollow outgrowths from its 



