ULTIMOBRANCHIAL BODY, FIFTH POUCH-BIRDS 587 



The only other embryo of this species in my possession of suf- 

 ficiently early . developmental stage to show remains of the fifth 

 visceral pouch is a specimen of approximately six and one-half 

 days. The pouch (fig. 6) is here on the verge of disappearing, 

 but can be definitely identified in the sections. The ultmio- 

 branchial diverticulum is a relatively large outgrowth of globu- 

 lar form, sharply differentiated on both sides of the embryo, 

 but that on the left considerably larger, as in the other species 

 considered. 



Chick. In the chick, judging from the available material 

 only, the early steps in the development of the fifth pouch and 

 the ultimobranchial diverticulum are not so clear as in the pre- 

 ceding species. In an 80-hour specimen there is a well-defined 

 diverticulum having a form and relationship quite similar to the 

 fifth pouch of the coot and the gallinule, in the early stages of 

 the pouch in these foniis. In a 90-hour embryo the correspond- 

 ing diverticulmii is relatively very large, appearing actually 

 larger than the fourth pouch itself, and is thicker-walled. In 

 two 100-hour embryos this evagination appears to have reached 

 its greatest development. It agrees in the main with the so- 

 called fifth pouch as that has usually been described for the 

 chick, forming a deep sac-like out-growth behind the fourth 

 pouch, its blind end directed caudad and slightly laterad towards 

 the ectoderm. It has a broader connection with the common 

 pharyngeal diverticulum than had the fifth pouch in either the 

 coot or the gallinule and is of more rounded form. It is also 

 situated relatively nearer the pharyngeal wall, a situation which 

 may have been brought about by a further lateral growth of the 

 fourth pouch. In one of these specimens the so-called fifth 

 pouch shows, on each side of the body, a slight indication of divi- 

 sion into a lateral and a medial lobe, similar to the division 

 shown in the four and one-half day tern, but the evidence in the 

 present case is not so clear. One gains the distinct unpression, 

 however, in examining this so-called fifth pouch in the 100-hour 

 and 96-hour embryos, that it represents something more than 

 merely the fifth pouch, such as it is characterized in the other 

 species discussed. From analogy with the relations of the fifth 



