THE AORTIC ARCHES. 179 



The fourth aortic arch, in the fourth branchial arch of the 

 tadpole, becomes the pulmo-cutaneous arch of the frog. It 

 retains its communication with the aorta for some time after the 

 dorsal part of the third arch has atrophied ; but before the end 

 of the first year, the part of the fourth arch above the origin of 

 the cutaneous artery loses its cavity and becomes solid, so that 

 the pulmo-cutaneous arch no longer opens into the aorta. 



The condition of the aortic arches is now that of the adult 

 frog (Fig. 81). 



Before leaving the branchial circulation, it should be noticed 

 that in other species of frogs the development of the blood- 

 vessels differs in some important respects from that described 

 above for the common English frog, Rana temporaria. In Rana 

 esculenta, according to Maurer, the efferent lacunar vessel of 

 each arch becomes connected at its ventral end with the truncus 

 arteriosus, and at its dorsal end with the aorta, before the afferent 

 branchial vessel is formed ; so that in this species of frog there 

 is a stage, prior to the formation of the gills, in which there is 

 a direct passage from the heart to the aorta. As the gills appear, 

 the afferent gill-vessel arises as an outgrowth from the ventral 

 end of the efferent vessel, with which it becomes connected, more 

 dorsally, through the gill-capillaries. During the time that the 

 tadpole is breathing by gills, the direct connection between the 

 ventral ends of the afferent vessel and the efferent vessel is lost, 

 but it is re-established at the time of the metamorphosis, after 

 which time the further changes are the same as in Rana tempo- 

 raria. A similar mode of development has been observed as an 

 exceptional occurrence in Rana temporaria itself, and there are 

 reasons for thinking that this plan of development, in which 

 there is, from the first, direct connection between the heart 

 and the aorta, through the aortic arches, and in which the whole 

 gill circulation is secondarily derived from this condition, is 

 more primitive than the mode of development usually seen in 

 Rana temporaria. This more primitive type of larval vessels is 

 comparable to the condition obtaining throughout life in the 

 branchial blood-vessels of Amphioxus. 



: 



The Dorsal Aorta and its Branches. 



There are at first two aorta?, one on either side of the body. 

 L'hese appear, as described above, as a number of isolated 



K 2 



