The pseudobranchial circulation in Amia calva. 125 



ceptiiig that iu several of the series the coniniissural branch from the 

 prespiracular to the postspiracuhir aortic arch could not be traced its 

 full length, and that the connection of the median ventral vessel with 

 the heart had shifted relatively somewhat further backward. Whether 

 the commissural vessel had, in most of these specimens, been actually 

 pinched otf, as it appeared iu the sections, or not, I am unable to 

 judge. 



In 9 mm embryos (Fig. 3) the conditions change somewhat and 

 begin to approach those described in 12 mm ones. In the first 

 branchial arch afferent and efferent arteries have been differentiated. 

 The other arches were not examined. There is a short truncus ar- 

 teriosus from which, posterior to the connection with the heart, a 

 lateral vessel arises, on each side, and sends branches in succession 

 to the second, third and fourth branchial arches. Anterior to the 

 heart a single median truncus gives origin, near its anterior end, to 

 the first pair of afferent branchial arteries, and then immediately 

 gives off, on each side, a single vessel which soon separates into the 

 post- and pre-spiracular arteries. Up to this age the prespiracular 

 artery runs in a nearly direct line from the short median ventral 

 vessel upward to the carotid, and it lies, at its dorsal end, considerably 

 in front of the spiracular outgrowth. In 9 mm embryos it has, on 

 the contrary, a short forward and then backward bend in its ventral 

 portion, and, dorsally, it first arrives opposite the spiracular cleft and 

 there turns forward, along the cleft, and, anterior to it, bends sharply 

 mesially to join the carotid. Thtj short dorsal section that thus lies 

 along and parallel to the cleft is large, and it is in relation to it that 

 the pseudobranch later develops, that organ developing in the tissues 

 immediately dorsal to the artery. At the point where the artery 

 reaches the spiracular cleft and bends forward along its lateral sur- 

 face, it sends backward, external to the hyomandibular, the com- 

 municating branch to the postspiracular aortic arch. As the pseudo- 

 branch lies anterior to the point where this communicating branch is 

 given off it thus certainly develops in relation to the prespiracular 

 aortic arch. 



The postspiracular artery, in 9 mm embryos, runs at first down- 

 ward and laterally, and then backward into the ventral end of the 

 gill cover. There it turns upward, and although probably interrupted 

 in its course by capillary spaces, that is, not being a continuous and 

 unbroken vessel, joins, near the hind edge of the hyomandibular, the 



