53G 



THE SUBIXTESTTXAL VEIX. 



really rhythmically contractile' the fact would be interesting as shewing 

 that this quality, which is now localised in the heart, was once probably 

 common to the siibintestinal vessel for its whole length. 



On the development of the cardinal veins (to be described below) 



considerable changes are effected in the 

 subintestinal vein. Its postanal section, 

 which is known in the adult as the caudal 

 vein, unites with tlie cardinal veins. On 

 this junction being effected retrogressive 

 changes take place in the prapanal section 

 of the original subintestinal vessel. It 

 breaks up in front into a number of 

 smaller vessels, the most important of 

 which is a special vein, which lies in the 

 fold of the e.piral valve, and which is 

 more conspicuous in some Elasmobranchii 

 than in Scyllium, in which the develop- 

 ment of the vessel has been mainly 

 studied. The lesser of the two branches 

 connecting it round the cloaca with the 

 caudal vein first vanishes, and then the 

 larger; and the two posterior cardinals 

 are left as the sole forward continuations 

 of the caudal vein. The latter then be- 

 comes prolonged forwards, so that the 

 two cardinals open into it some little 

 distance in front of the hind end of the 

 kidneys. By these changes, and by the 

 disappearance of the postanal section of 

 the gut, the caudal vein is made to 

 appear as a supraintestinal and not, as 

 it really is, a subintestinal vessel. 



From the subintestinal vein there is 

 given off a branch which supplies the 

 yolk-sack. This leaves the subintestinal 

 vein close to the liver. The liver, on 

 its development, embraces the subintestinal vein, which then breaks 

 up into a capillary system in tlie liver, the main part of its blood 

 coming at this period from the yolk-sack. 



The portal system is thus established from the subintestinal vein; 

 but is eventually joined by the various visceral, and sometimes by the 

 genital, veins as they become successively developed. 



The blood from the liver is brought back to the sinus venosus by 

 veins known as the hepatic veins, which, like the hepatic capillary 

 system, arc derivatives of the subintestinal vessel. 



Ficj. 367. Section thkough 



THK TRUNK OF A ScYLLIUM EMIJIIYO 

 SLIGHTLY \OUNGEK THAN 28 F, 



sp.e. spinal canal ; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 .(■. sub-notochordal rod ; ao. aorta ; 

 nip. muscle-plate; 7itp'. inner layer 

 of muscle-plate already converted 

 into muscles; TV. rudiment of 

 vertebral body; st. segmental 

 tube; ,sd. segmental duct; spA\ 

 spiral valve; i'. subintestinal vein; 

 |).o. primitive generative cells. 



1 J. Miillcr holds that this sack is not rhythmically contractile. 



