652 



THE SUBINTESTINAL VEIN. 



-p.o 



rp.r. 



dilated portal sinus of Myxine is probably also part of it ; and if this is 

 really rhythmically contractile 1 the fact would be interesting as shewing that 

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

 to the subintestinal 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 the cardinal veins. On 

 this junction being effected retro- 

 gressive changes take place in the 

 praeanal section of the original sub- 

 intestinal 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 spiral valve, and which is 

 more conspicuous in some Elasmo- 

 branchii than in Scyllium, in which 

 the development 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 car- 

 dinals are left as the sole forward 

 continuations of the caudal vein. 

 The latter then becomes 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 dis- 

 appearance 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 



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



FIG. 367. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM 

 EMBRYO SLIGHTLY YOUNGER 



THAN 28 F. 



sp.c. spinal canal; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots; ch. notochord ; 

 x. sub-notochordal rod ; ao. aorta ; 

 mp. muscle plate; ;;//'. inner layer 

 of muscle-plate already converted 

 into muscles; Vr. rudiment of 

 vertebral body; st. segmental 

 tube ; sd. segmental duct ; sp.v. 

 spiral valve; v. subintestinal vein ; 

 p.o. primitive generative cells. 



