214 PROFESSOR MARSHALL AND EDWARD J. BLES. 



the eyes. Behind the gills the aortse bend towards the median plane, 

 at first sharply, and then more gradually; the two vessels meet 

 a little behind the level of the first nephrostomes, and unite to form 

 the median dorsal aorta, which extends back beneath the notochord 

 somewhat further than at the earlier stage. 



Above the pharynx each dorsal aorta receives five vessels ; of these 

 the first, second, and fifth (E M, E H, and E F. 3), belonging respec- 

 tively to the mandibular, hyoidean, and third branchial arches, are 

 small, short, and blind distally ; while the other two (E F. 1 and E F. 2) 

 are of large size, and, as already noticed, return to the aorta the 

 blood from the gills on the first and second branchial arches. 



Shortly before the two aorta? unite to form the median dorsal 

 aorta, each is in connection with a glomerulus, G, which is now a 

 well-marked and sacculated diverticulum from the ventral and outer 

 surface of the aorta. 



h. The Feins. — The liver is now assuming more definite form, and 

 the hepatic and vitelline veins are distinct from one another. Of 

 the hepatic veins (Fig. 6, V H) the right is apparently constantly of 

 larger size than the left. The Cuvierian veins, V D, are considerably 

 larger than before, and each divides dorsally into anterior and posterior 

 cardinal veins, of which the latter are of large size, and surround the 

 tubules of the head kidney in an intricate fashion. 



5. General Considerations. 



There is little to call special attention to at this stage, except 

 the very significant arrangement of the vessels in the mandibular, 

 hyoidean, and first branchial arches. In the mandibular arch the 

 vessels are just forming, and are now in a condition comparable to 

 that of the hyoid vessels at 4J mm. In the hyoid arch the vessels 

 have reached their full development, and have already begun to 

 undergo degenerative changes. In the third branchial arch the 

 actual condition is closely comparable to that of the hyoid arch. A 

 lacunar efferent vessel is present, as also are diverticula from the 

 aorta and truncus arteriosus, these three elements being perfectly 

 independent, and indeed widely separate. While, however, the 

 hyoidean vessels are commencing to degenerate, those of the third 

 branchial arch advance steadily in development during the succeed- 

 ing stages. 



