vi AETEEIAL SYSTEM 407 



here again it would appear from later investigations that the 

 definitive artery is merely a surviving and enlarged representative 

 of a number of original supply vessels (Evans, 1909). 



Eepresentatives of the series of intersegmental arteries are 

 recognizable in various Vertebrates in the hinder part of the head- 

 region. Thus in Lacerta (van Bemmelen, Hochstetter, 1906) three 

 have been detected in the head-region. Of these the first two dis- 

 appear while the third becomes prolonged forwards immediately 

 beneath the brain to become continuous with the internal carotid at 

 the level of the mid-brain. These prolongations fiise together in the 

 mid-line and form the basilar artery. 



A similar basilar artery continuous with the internal carotids is 

 of common occurrence in Vertebrates though the relations of its 

 paired fore-runner to the series of intersegmental arteries differ in 

 different forms. 



The vertebral artery of Sauropsida is in its origin intimately 

 related to the intersegmental arteries. Thus in Lacerta the inter- 

 segmental vessels posterior to the subclavian become connected 

 together by a longitudinal anastomotic vessel, which persists and 

 forms the cervical portion of the vertebral artery. In Snakes the 

 two similarly -arising vessels apparently usually undergo fusion 

 together in their anterior portion while farther back the unpaired 

 condition is reached by the disappearance of the rudiment on the 

 right side (Hochstetter, 1906). 



The arterial blood - supply of the urino - genital organs is also 

 generally provided by branches from the intersegmental arteries of 

 the embryo. 



Mesenteric Arteries. — The digestive tract receives a varying 

 number of branches from the dorsal aorta which may be at first 

 paired and undergo fusion secondarily or may be unpaired from the 

 beginning. In those vertebrates which have a bulky yolk-sac a 

 pair of these are precociously developed as vitelline arteries. In 

 some cases there is a remarkable relation between the chief mesenteric 

 artery (coeliac) and the pronephros. Thus in Lepidosiren a connexion 

 becomes established between the blood-spaces of the right pronephros 

 and those of the gut wall, and the branch of the dorsal aorta which 

 supplies the right pronephros persists as the root of the definitive 

 coeliac artery. In such Teleostean fishes as the Trout the connexion 

 with the pronephric arterial supply is only temporary, a new 

 anastomotic channel arising farther back between dorsal aorta 

 and mesenteric artery which remains as the definitive root of the 

 latter vessel. 



Venous System. — As an example of the development of the 

 venous system in a holoblastic Vertebrate we will take that of 

 Lepidosiren (Eobertson, 1913). 1 



1 For comparison with Lepidosiren, an account of the development of the vascular 

 system of Ceratodus will be found in Kellicott (1905). 



