500 FRANK REAGAN 



ridge. After coursing dorsally and laterally 30m from its most 

 median indication, this ridge abruptly gives off, continuous with 

 its extremity a fifth vessel which, after a very short ventro-caudal 

 course makes a right angle turn in a dorso-caudal direction, curv- 

 ing at the same time laterally. After passing between the much 

 attenuated glandule thyroidienne and the broad 'pre-pulmonic 

 caecum, the vessel curves in a dorso-median direction and joins 

 the dorsal aorta not far from the dorsal lumen of the fourth arch. 

 But just before uniting with the dorsal aorta, the fifth vessel 

 gives off two branches neither of which has so great a diameter 

 as that of the fifth vessel at this point. The smaller of these two 

 branches passes in a caudal direction and joins the pulmonic 

 arch near its dorsal lumen. The larger branch lies practically 

 opposite the branch just described; it passes anteriorly to join 

 the systemic arch just ventral to the union of that vessel with the 

 aortic root. 



The conditions of this embryo are suggestive of those seen in 

 Lehmann's fig. 12. Their points in common are: that in each 

 case the fifth arch has a branch communicating with the pulmonic 

 arch; these communications leave (or perhaps enter) the fifth 

 arch at about the same angle; the anterior portion of each pul- 

 monic connection is relatively small, while the posterior portion 

 of each is large and flaring. The last fact may be suggested as 

 indicating the connection to have originated from the pulmonic 

 arch. My figure differs from that of Lehmann, in that the fifth 

 vessel of my fig. 8 is more intimately connected with the ventral 

 aorta; the portion entering the aortic root is much larger than 

 the pulmonic branch. These conditions indicate that Lehmann 

 was correct in interpreting the pulmonic connection in her fig. 

 12 as a branch of the fifth arch. My fig. 9 is made from a recon- 

 struction of a 9 mm. embryo. The rudiment of the fifth vessel 

 has a systemic connection, but is in no way connected with the 

 pulmonic arch. These conditions also point to the correctness 

 of Lehmann's conclusions. 



Stages later than 9 mm. exhibit irregularities in abundance, 

 but they gradually become less typical. In a 10 mm. embryo, 

 a pulmonic arch was seen as a Y-shaped vessel. A long finger- 



