412 EMBBYOLOGY OF THE LOWEK VEETEBEATES ch. 



is known by the same name (Fig. 190, D, s.i.v"). It will be realized 

 that this anterior section of the subintestinal vein is developmentally 

 of a different nature from the posterior portion for it is formed by a 

 short-circuiting of the blood-stream through the vitelline network, 

 while the posterior portion represents rather the conjoined hinder 

 ends of the paired vitelline veins (Fig. 190, C and D). This differ- 

 ence in development is no doubt purely secondary and we may take 

 it that the later condition, where the subintestinal vein is continuous 

 right forwards to the heart, represents the really primitive condition 

 of this vein in evolution. The continuity of the subintestinal vein 

 at its front end with the heart is brought about in ontogeny through 

 blood-sinuses which make their appearance in the liver (stage 31). . 



The right vitelline vein and the anterior, secondarily formed, 

 portion of the subintestinal vein now gradually disappear (about 

 stages 32 - 35). The left vitelline vein ceases to form a continuous 

 channel over the surface of the liver to the heart, so that the blood 

 in it is forced to traverse the system of blood sinuses within the 

 liver. In other words the whole of the blood which streams forwards 

 in the subintestinal vein is diverted along the persisting left vitelline 

 vein into the network of blood spaces in the liver. Subintestinal 

 vein and left vitelline vein have thus come to constitute the hepatic 

 portal vein. The latter becomes complicated by a branch sprouting 

 out from the front end of its subintestinal portion. This branch 

 spreads round the alimentary canal along the line of the spiral valve, 

 fusing with the subintestinal vein at each point of intersection 

 (Fig. 190, F). As the liver increases in length a special supply 

 channel from the portal vein lengthens out along its left side, giving 

 off numerous branches into the liver substance (Fig. 190, F). From 

 this the blood drains by numerous efferent vessels into the intra- 

 hepatic portion of the posterior vena cava. 



Caudal Vein. — The post-anal portion of the subintestinal vein 

 was left (p. 408) at a stage when its anterior bifurcated portion was 

 continuous on each side not only with the pre-anal portion of the 

 same vein but also with the dorsal aorta and the posterior cardinal. 

 As development goes on the first two of these connexions disappear 

 so that the post-anal subintestinal vein is now continuous anteriorly 

 only with the posterior cardinal. With the atrophy of the post-anal 

 gut, it comes to lie immediately beneath the caudal portion of the 

 dorsal aorta and is now known as the caudal vein, its anterior 

 forked portion forming the hinder ends of the renal portals. 



The veins which have been described constitute the main trunks 

 of the venous system ; in the later stages of development a number 

 of other important vessels appear which are indicated in the 

 diagrams. The subclavian vein (Fig. 190, d, s) appears about stage 

 31 + leading from the pectoral limb into the pronephric sinus. As 

 this sinus atrophies the point of opening of the subclavian vein comes 

 to be situated on the anterior cardinal vein (Fig. 190, e, s). From 

 the subclavian vein a small lateral cutaneous vein (l.v) passes back 



