398 EMBRYOLOGY OF THE LOWER VERTEBRATES oh. 



cr.I 



a duct of Botallus forming a connexion between the pulmonary 



artery and the aortic root 

 m IV V VI A just as is shown in Fig. 



lU 186, B (d.B) for the Uro- 



dele amphibian. In other 

 cases it may persist as a 

 ligamentous vestige, as is 

 the case on the left side in 

 Tropidonotus. 



As a rule the portion of 

 aortic root lying between 

 arches III and IV disap- 

 pears during development 

 but in most Lizards (not 

 in Chameleons and Moni- 

 tors) it persists in the 

 adult, so that in a dis- 

 section arches III and IV 

 appear to run into one 

 another peripherally. 



In Monitors (Vara- 

 nidae), in correlation with 

 the elongation of the neck, 

 arches III and IV become 

 widely separated from one 

 another and the interven- 

 ing portion of ventral aorta 

 shows a corresponding 

 lengthening both in its 

 paired (common carotid) 

 and its unpaired (primary 

 carotid) portions. 



In Birds (Fig. 187a, D) 

 arch IV completely dis- 

 appears on the left side 

 and with it the portion of 

 the left aortic root lying 

 posterior to it. Conse- 

 quently in the adult Bird 

 there is only a single sys- 

 temic aortic arch and it 

 passes down the right side 

 of the body. 



Elasmobeanchii. — In 

 the Ichthyopsida, as we 

 should expect, the depart- 

 ures from the primitive 

 scheme are lesspronounced: 



KC.H- 



Fig. 188. — Illustrating the modification of aortic arches 

 III-VI during ontogeny in Scyllium, according 

 to Dohrn. 



A, Dorsal aorta; a/, afferent branchial; an, anastomotic 

 vessel ; ef, efferent branchial ; v. A, ventral aorta ; v.c, visceral 

 clefts ; III, IV, V, VI, aortic arches. 



