318 CHARLES EUGENE JOHNSON 



5. The fifth visceral pouch seems to disappear soon after it 

 attains its greatest development, which in Chelydra was found to 

 be in embryos of 7.5 mm. to 9 mm. greatest length. 



6. A conspicuous ultimobranchial vesicle is usually present 

 on each side in the early stages, but the one on the right, as a rule, 

 soon reaches hmitations in growth and becomes greatly exceeded 

 in size by its fellow of the opposite side. The body on the 

 right may apparently at times be wholly lacking. Where both 

 are present, they appear to undergo parallel differentiation, at 

 least up to the time of hatching. The relatively huge dimen- 

 sions sometimes attained by the dominant ultimobranchial 

 vesicle is a striking feature. 



7. In turtles the fourth and fifth visceral pouches and the 

 ultimobranchial diverticulum originate in a single conspicuous 

 evagination from the lateral pharyngeal wall. In this evagina- 

 tion the fourth pouch is the first to be differentiated; next ap- 

 pears the ultimobranchial diverticulum, and lastly the fifth pouch 

 may be distinguished, which is closely associated with the ultimo- 

 branchial diverticulum and is very small. 



8. The fourth pouch and the ultimobranchial diverticulum 

 become separated as a unit from the pharynx, but remain con- 

 nected with each other until a comparatively late stage in their 

 development. 



9. The fourth pouch in subsequent development exhibits 

 more or less of a tendency toward cyst formation. This seems 

 to be manifested earlier in Trionyx than in the other two genera 

 studied; but in later stages very large cysts, relatively speaking, 

 were found in connection with the fourth pouch in Chelydra. 

 In Chelydra such tendency was observed also in connection with 

 the parathyreoid III. The significance of the cysts is not 

 clear. 



10. At the time of hatching the thymus is a rather voluminous 

 body of oblong shape, and parathyreoid III is a relatively small 

 rounded body which is more or less deeply imbedded in the 

 caudal portion of the former gland. 



11. Parathyreoid IV, similar in size and shape to the para- 

 thyreoid III, is usually found in close association with or partly 



