230 PROFESSOR MARSHALL AND EDWARD J. BLES. 



The efferent vessel (Figs. 11 and 12, E F.3) runs along the 

 anterior border of the gill to its base, and then turns up to the 

 dorsal aorta; below the gill it is continued ventralwards a short 

 distance, ending blindly. 



/. The Fourth Branchial Arch. — The vessels of this arch are in 

 almost exactly the same condition as at the last stage, except that 

 the pulmonary artery is rather larger than before. The third and 

 fourth branchial efferent vessels join the dorsal aorta so close 

 together that they appear almost to unite before reaching it. 



At this stage pulmonary veins are present as irregular spaces 

 along the inner surfaces of the lungs, in the solid mesoblast connecting 

 the lungs with the cesophagus. These pulmonary veins do not reach 

 the heart, and there is, as we have seen, no trace of a division of the 

 auricle as yet. 



g. The Dorsal Aorta. — The relations of the dorsal aortse are almost 

 precisely the same as at 6| mm. The course of the anterior arteries 

 is shown in Fig. 11, and the only additional vessels that require 

 mention are the basilar arteries (Fig. 11, A B), which arise from the 

 outer ends of the posterior or dorsal infundibular commissure, and 

 run back along the ventral surface of the brain and spinal cord, not 

 far from the median plane. 



h. The Veins. — We have nothing special to notice about the 

 veins. The posterior vena cava is commencing to form, but will be 

 more conveniently described at the next stage. Both anterior and 

 posterior cardinal veins are of large size, the anterior cardinal being 

 formed by the union, behind the ear, of a jugular vein returning 

 blood from the brain and the dorsal part of the head, and a facial 

 vein which lies superficially along the side of the head, ventral to 

 both eye and ear. 



5. General Considerations. 



These, as already noticed, will be postponed till after the next 

 stage (12 mm.) has been described. The most interesting points 

 arising in connection with the blood vessels at the present stage are 

 the almost complete disappearance of the vessels belonging to the 

 hyoid arch, the formation and relations of the lingual artery, and 

 the first commencements of the internal gills. Of general interest 



