No. I.] MORPHOLOGY OF VERTEBRATE HEAD. 91 



ascending the arch the vessel leaves the horizontal plane, to the 

 point where, in turning towards the aorta, it becomes again hor- 

 izontal. Supposing, as seems probable, that the commissure 

 which unites the vein of the hyoid arch with the mandibular 

 artery, is really the homologue of a commissure of the gill 

 arches, this assumption would not necessarily involve the fur- 

 ther assumption that its position is identical with that of the 

 final commissures of the arches, since these must represent a 

 reduction of five or more primary commissures, extending in all 

 over quite a wide space. Yet this second assumption would be 

 necessary in order to argue from the position of the hyoid com- 

 missure to the homologies of structures lying above or below it. 



At this stage, in which the gill arches possess several com- 

 missures, the walls of the mandibular artery have become very 

 thin below the commissure that connects this artery with the 

 hyoid vein, and it is difficult to follow the artery in its ventral 

 portion until it again becomes distinct on turning towards the 

 axial line of the body. Retrogressive development is evidently 

 taking place. The hyoid vein and the anterior vein of the first 

 branchial arch are now prolonged ventrally towards the inner 

 limit of the first gill cleft. Here the branchial vein is continued 

 in a small vessel with thick walls, which, passing below the 

 inner opening of the cleft, unites with the ventral prolongation 

 of the hyoid vein, and is then further continued over the dorsal 

 wall of the hyoid artery to unite with the ventral end of the 

 mandibular artery, which has now lost its connection with the 

 aorta (Fig. n, *, PI. ). Maurer (No. 12) says that he has seen in 

 several series of Acanthias embryos a ventral continuation (as 

 in the Teleosts) of the first gill vein, which united with the first 

 arterial arch (mandibular). He does not mention a union with 

 the hyoid vein at the same point. Yet this anastomosis seems 

 to me of interest, since it gives an intermediate stage in the 

 successive reductions by which one passes from the hyoid circu- 

 lation in the Selachians to that of the Teleosts and Amphibians. 



In Selachians, at an early stage, a single aortic arch exists in 

 each of the mandibular, hyoid, and successive branchial arches. 

 In the branchial arches this vessel soon divides into three, the 

 artery, and the anterior and posterior branchial veins. In the 

 hyoid arch the primary vessel divides into but two, the artery 

 and the posterior vein, while in the mandibular arch the vessel 

 remains undivided. 



