586 



CHARLES EUGENE JOHNSON 



it is well differentiated and is situated entirely mediad of the fifth 

 pouch, fomiing an evagination of the common pharyngeal di- 

 verticulmn. This common pharyngeal diverticulum, as a result 

 apparently of its further outgrowth and the consequent narrow- 

 ing of its connection with the pharynx, together with the gradual 

 reduction of the fifth pouch, now has more the appearance of 

 being the fourth pouch proper, with the fifth pouch and the ulti- 

 mobranchial diverticulum forming merely secondary outgrowths 

 from its wall. 



ul. h 



a a 6 



a. a A 



Fig. 1 Horizontal section through the region of the fourth and fifth visceral 

 pouches and the ultimobranchial diverticulum of an embryo Forster's tern of 

 four and one-half days. A.a.J^,5,6, fourth, fifth and sixth aortic arches; v.p.J^.S, 

 fourth and fifth visceral pouches; ul.h., ultimobranchial body. X 112. 



Forster's tern. The youngest embryo of this tern (Sterna 

 forsteri Nutt.) in my collection is one of about four and one-half 

 days. In this specimen the fifth pouch is well developed (fig. 

 1). At its junction with the pharyngeal wall is a second clearly 

 defined diverticulum, present on both sides of the embryo. This 

 is the anlage of the ultimobranchial body. It differs from the 

 corresponding diverticulum in the coot and the gallinule, in 

 making its appearance somewhat earlier and in being more 

 closely associated with the fifth pouch; but it agrees in these re- 

 spects with the conditions presented by the four-day duck 

 (Rabl). 



