THE MAMMALIAN FIFTH ARCH 499 



this ridge the fifth vessel has its origin. It curves freely in a 

 latero-dorsal and at the same time, caudal direction (fig. 7), 

 passing between the glandule thyroidienne and the 'prepulmonic 

 caecum.'* From this point, the fifth vessel curves rather abruptly 

 towards the median line. Just ventral to its union with the aortic 

 root, the vessel increases greatly in size; this increase is more 

 prominent on the posterior surface (figs. 1 and 7). Opposite 

 this prominence^ is a slight outbulging on the anterior face of the 

 pulmonic arch. The mesenchyme is continuous between the 

 two vessels (fig. 1). The dorsal lumen of the fifth vessel is situ- 

 ated about midway between the systemic and pulmonic arches, 

 (figs. 1 and 7). 



The left side of the same embryo (fig. 10) has only the rudiments 

 of a fifth arch. The vessel seems to have lost its original and 

 most ventral connection, and has formed a second one higher up 

 on the fourth arch. The portion between this and the original 

 connection has partly degenerated. There are dorsal connec- 

 tions with the adjoining arches somewhat similar to those in fig. 

 8. (In both instances the pulmonic connection is the smaller.) 

 These dorsal branches have persisted and probably represent the 

 connecting sinus found by Tandler in the rat. I have found 

 dorsal connections between the fourth and pulmonic arches in 

 about thirty-five instances in which the dorsal connection varies 

 from a very slender to a broad connection where the fourth and 

 pulmonic arches apparently come in contact (fig. 14). The very 

 slender connections seldom have direct connection with the dorsal 

 aorta. 



The 9 mm. sagittal series Number 1299 of the University of 

 Chicago collection is of much interest because of a complete fifth 

 vessel (fig. 8). Twenty microns lateral to the plane of diversion 

 of the third and fourth arches, there is seen on the ventro-caudal 

 surface of the ventral aorta (somewhat similar in position to the 

 vessel on the right side of the preceding embryo) a prominent 



•• For the significance of this term, see part 2. 



* On the postero-dorsal surface of the systemic arch is also a small prominence 

 of about the same size. These probably indicate the beginning of branches of the 

 fifth arch. 



