DEVELOPMENT OF THE BLOOD VESSELS IN THE FROG. 221 



arch, and which at present has no communication with any other 

 vessel. 



g. The Dorsal Aorta. — The course and relations of the dorsal aortse 

 are shown on the right side in Fig. 7, and from above in Fig. 9. 

 In the latter figure the gills and gill arches of the right side are drawn 

 from a section at a level somewhat ventral to that represented on the 

 left side, where the entire course of the aorta is shown. The two aortse 

 are furthest apart opposite the first branchial cleft ; in front of this 

 level they converge very rapidly to the hinder border of the infun- 

 dibulum ; here they turn sharply forwards and slightly outwards, 

 and run as the internal carotid arteries along the sides of the brain, 

 passing dorsal to the eye stalks. The two aortse are connected 

 together by two transverse commissural vessels, one lying above and 

 one below the infundibulum (Figs. 7 and 9, AM, A M'). Behind 

 the first branchial clefts the two aorta? run backwards along the roof 

 of the pharynx (Fig. 10, A). They converge at first rapidly, then 

 more gradually, and unite about opposite the level of the first 

 pair of nephrostomes of the head kidneys (Fig. 9). Immediately 

 behind the point of union there is present on each side of the 

 aorta a large glomerulus (Fig. 9, G), lying opposite almost the whole 

 length of the head kidney. 



Each dorsal aorta receives in the pharyngeal region, as already 

 noticed, six efferent branchial vessels. Of these the first or man- 

 dibular vessel (Figs. 7 and 9, E M) is very slender, is in communica- 

 tion distally with the mandibular vein (Fig. 7, V M), and has no 

 corresponding afferent vessel. The second or hyoid efferent vessel 

 (Figs. 7 and 9, E H) is at this stage an extremely short blind 

 diverticulum. The third and fourth are the large efferent vessels 

 of the first and second branchial arches, which transmit to the aorta 

 the blood from the large external gills of these arches, and which are 

 in connection with the heart through the capillaries of these gills. 



The last two — the fifth and sixth — are the efferent vessels of the 

 third and fourth branchial arches, which at present have no com- 

 munication with the heart. The pulmonary artery is present at this 

 stage, but if any blood flows along it, this must be blood from the 

 dorsal aorta. 



h. The Veins. — We have but little to say at this stage concerning 

 the veins, which we have studied in far less detail than the artories. 



