398 EMBRYOLOGY OF THK Ln\VKI! VEKTEBKATES MI. 



a duct of Botallus form ing a connexion between the pulmonary 



artery and the aortic root 

 IV V VI A. just as is shown in l-'ig. 



A. L 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. 



I n Birds (Fig. 187 A, D) 

 arch IV completely dis- 

 appears on the left side 

 and with it the portion <f 

 the left aortic root lyin^ 

 posterior to it. Conse- 

 <|iiently in the adult I'.ird 

 there is only a sin-lr 

 l.einir aortic ,-nvli and it 

 passes dov.ii the ri-ht side 

 of the hudy. 



Kl..\sM(H;i;.\Nci!ll. In 

 the lchth\opsida. as we 

 should expect, the tleparl 

 ures I'nun the prinniive 

 sehemeare le^sprmiouiiced: 



wc.ff. 



FKJ. 188. Illustrating tli- iii.liii."iii,,n..ra..rti. an hr- 

 ill VI .hum- MiitoKi-n\ in M-i/Hiiim, an-.. i, Un- 

 to Doll Ml. 



UK ln:il ; OH, ;in;istoni.,li.- 



vt'KKel ; e/, efferent i.ia 

 elefta; III, IV, V, 



