400 EMBRYOLOGY OF THE LOWEB VERTEBRATES oh. 



Cyclostomata. — In the Lamprey it should be noted that 

 according to Dohrn (1888) an aortic arch corresponding to 

 aortic arch I of Gnathostomata makes its appearance and then 

 disappears .again. In Myxinoids the most important feature is that 

 in them the number of aortic arches reaches ' its maximum for 

 Craniata — up to 14 in Bddlostorna. 



Ckossopterygii. — Our knowledge is in this case very incomplete. 

 The chief peculiarity (Graham Kerr, 1907) is that, correlated with 

 the large size of the external gill belonging to arch II, which forms 

 the sole respiratory organ during early stages of larval life, aortic 

 arch II makes its appearance relatively early and the development 

 of the other aortic arches is postponed. Distinct vestiges of 

 aortic arch I make their appearance. The succeeding aortic arches 

 remain small for a prolonged period. Aortic arch VI becomes much 

 enlarged in its ventral part in correlation with the fact that it 

 supplies the pulmonary artery. 



Actinopterygii. — In the Teleostean fishes and in the Ganoids 

 that approach most nearly to them (Lepidosteus — F. W. Miiller, 

 1897 ; Amia — Allis, 1900) complicated changes, which need not be 

 detailed, take place in arches I and II in relation with the blood 

 supply of the pseudobranch which in these fishes (p. 159) comes to 

 he on the inner surface of the operculum. 



Dipnoi. — In Lepidosiren (Robertson, 1913) aortic arches I and II 

 never become complete well-developed vessels •: they are vestigial 

 and their ventral portions do not appear to develop at all. The 

 remaining four aortic arches are well developed, each passes out into 

 an external gill and in each an intercalary piece becomes developed 

 to short-circuit the blood-stream at the time the external gills 

 atrophy. In Ceratodus and Protopterus efferent downgrowths -make 

 their appearance as in Elasmobranchs but they remain connected 

 with the dorsal portion of their own aortic arch and do not undergo 

 fusion at their ventral ends so that the condition in the adult 

 departs less from the primitive than it does in the Elasmobranch. 



Amphibia. — In Urodela the arrangement of aortic arches 

 (Fig. 186, B) closely resembles that in Lung-fishes. Arches I and 

 II are reduced : the latter in fact is according to Maurer (1888) no 

 longer to be detected at all in the case of Triton. Arches III 

 IV and V are prolonged outwards into external gills and in each 

 case a short-circuiting piece becomes intercalated as the external 

 gills lose their functional activity. According to Maurer the inter- 

 calary portion makes its appearance as a downgrowth from the 

 dorsal or efferent limb of the aortic arch but this may perhaps be 

 doubted in view of the fact that in Lung-fishes the corresponding 

 piece of vessel develops by the widening out of pre-existing chinks 

 (Robertson, 1913). 



Amniota. — Arch IV along with the aortic root into which it 

 passes forms the main systemic aorta on each side. That of the 

 left side is connected, in correlation with the spiral twist of the 



