200 PROFESSOR MARSHALL AND EDWARD J. BLES. 



reaches almost to the level of the aorta, A; its ventral end is 

 difficult to trace, but can be followed with tolerable certainty as 

 far as is shown in Fig. 4, and with more doubt right down to 

 the floor of the pharynx. Along its whole length it has the appear- 

 ance of an irregular lacunar space between the mesoblast cells, and 

 possessing no true walls of its own. In places, however, the meso- 

 blast cells immediately surrounding it are more closely arranged 

 than elsewhere, as though tending to form a definite wall. 



Opposite the dorsal end of this lacunar vessel, which we shall 

 find represents the efferent branchial vessel of the hyoid arch, there 

 is a very small diverticulum of the dorsal aorta (Fig. 4, E H). This 

 lies to the inner side of the lacunar vessel, and has as yet no con- 

 nection with it. 



c. The First Branchial Arch. — In this arch two vessels are present, 

 a posterior and smaller afferent vessel, and an anterior and larger 

 efferent one. 



The afferent vessel (Fig. 4, A F') is small in size and irregular 

 in shape ; it is widest at its dorsal end, which lies opposite the 

 base of the external gill, and sends a small diverticulum outwards 

 into the gill. This diverticulum is lacunar, i.e., it has rather the 

 appearance of an irregular channel between the mesoblast cells of 

 the gill than of a vessel with definite walls. In some specimens the 

 diverticulum ends blindly ; in others it appears to be connected by 

 very narrow lacunar channels with the efferent vessel. Below the 

 base of the external gill the afferent vessel continues straight down- 

 wards in the side wall of the pharynx ; it gradually diminishes 

 in size, and ends blindly some little distance from the truncus 

 arteriosus, with which, however, it is connected by a solid string of 

 mesoblast cells, more definitely arranged, and staining more deeply 

 than the general mesoblast of the arch. 



The efferent vessel (Fig. 4, E F') lies anteriorly to the afferent, 

 and is of much larger size. Its ventral end lies very close to the 

 truncus arteriosus, and in a straight line with it ; there is, however, 

 a distinct interval between the two vessels, and the lower end of the 

 efferent vessel is blind. Passing upwards in the wall of the pharynx 

 the efferent vessel gets rapidly larger ; it reaches its greatest size 

 opposite the external gill, into which it extends, and within which in 

 some specimens, as already noticed, it is connected with the afferent 



