34 CHARLES EUGENE JOHNSON 



it, between the fourth and fifth pouches is the fifth visceral 

 arch (V.a.V). KalHus ('05) had previously noted the sixth 

 mesenchymal ridge and had observed a small blood-vessel in it, 

 but was not prepared to state whether or not this vessel repre- 

 sented an aortic arch. Rabl, however, found the rudimentary 

 sixth aortic arch in close association with this ridge, sometimes 

 in it, sometimes at one side, and his conclusion that the ridge is 

 the sixth visceral arch rests in part upon this fact. 



The youngest embryo in my series is one of forty-six hours. 

 At this stage a division of the so-called fifth pouch is not dis- 

 tinguishable, although this diverticulum is prominently devel- 

 oped and opens more directly into the pharyngeal cavity than is 

 the case in Rabl's duck embryo of four days and seven hours, in 

 which the fifth and fourth pouches are at the end of a common 

 outpocketing of the pharyngeal wall. In the forty-six-hour grebe 

 the diverticulum of the fourth pouch extends ventrally beyond 

 the fifth pouch as a blind pocket. The first four aortic arches 

 are present in full length from truncus arteriosus to aortic root 

 (fig. 10). The fourth is the smallest, and the left one is con- 

 siderable smaller than the right. Shortly after leaving the aortic 

 root the fourth arch on each side gives off a branch which courses 

 along the posterior wall of the fifth visceral pouch and can be 

 traced ventrally about half the distance to the truncus arteriosus. 

 Near its origin each of these vessels in turn has a small, short 

 branch directed so that, if extended, the vessel would pass ven- 

 trally along the border line between the fourth and fifth divertic- 

 ula. From the position of these vessels it is evident that the 

 posterior larger one is the sixth aortic arch and the anterior im- 

 perfectly developed branch is the fifth. So far as the evidence 

 from this stage goes, it points to a condition of these vessels 

 entirely in accord with that in the duck. The available stages 

 however, are not adequate for ascertaining the relation of the 

 sixth aortic arch to the corresponding pouch and visceral arch 

 during all of the subsequent development of these structures. 



An embryo of four and a half days presents what appears on 

 casual examination to be an undivided diverticulum correspond- 

 ing to the fifth pouch of the forty-six-hour stage. A more careful 



