ULTIMOBRANCHIAL BODY, FIFTH POUCH-BIRDS 591 



but the two divisions thus fomied are united at the base and 

 connect with, the pharynx through a common passage. The 

 fifth pouch is a small elongate diverticulmn arising from the 

 lateral side of the ultimobranchial outpouching, i.e., where this 

 outpouching joins the coimiion pharyngeal stalk. It extends 

 parallel with the fourth pouch and comes into close contact with 

 the ectoderm. The right pouch is considerably larger than the 

 left. On each side the pouch projects through the vascular ring 

 formed by the fifth and sixth aortic arches, the fifth arch being 

 very short and joining the sixth immediately above and below 

 the pouch. The disparity in size between the fourth and the 

 fifth pouch in the turtle at this stage is greater than that be- 

 tw^een the corresponding pouches in the four and one-half day 

 gallinule. The ultimobranchial diverticulmn on the other hand 

 is relatively very much larger. Its place relations, however, are 

 practically identical with those of the ultimobranchial diverticu- 

 lum in the birds described. 



The fonnation of the fifth visceral pouch and the ultunobran- 

 chial diverticulum in birds and in reptiles is essentially the same. 

 In the reptiles, as represented by the turtles, these evaginations 

 are differentiated simultaneously with the fourth visceral pouch 

 from a common pharyngeal outpouching. This may possibly 

 be explained by the relatively large pharyngeal area concerned 

 with the fonnation of the ultunobranchial body. In the birds 

 the fourth and fifth pouches appear first. The ultunobranchial 

 body is somewhat retarded and its anlage involves a relatively 

 smaller area. The fifth pouch of birds, like that of both lizards 

 and turtles, is small and unimportant; apparently it only rarely 

 gives rise to thymus tissue. The variations in its developmental 

 details are probably governed by those of the greater and more 

 unportant diverticula of the fourth pouch and the ultimobranchial 

 body. 



