344 OUTLINES OF CHORDATE DEVELOPMENT 



veins are considerably enlarged as the ductus Cuvieri, which 

 turn inward and downward and pass into the sinus venosus 



(Fig. 137). 



From the upper end of each ductus Cuvieri an outgrowth 

 extends posteriorly as the rudiment of the posterior cardinal 

 vein, which passes along the Wolffian duct (see below), finally 

 reaching nearly to the base of the tail. The posterior cardinal 

 veins receive the intersomitic or intersegmental veins, except 

 the first three, and the vessels of the nephros (mesonephros, see 

 below). The veins of the fore-limbs also discharge into the 

 posterior cardinals near the ductus Cuvieri. The anterior 

 and posterior cardinal veins are consequently the chief somatic 

 veins of the early embryo, and it should be noted that all of 

 the somatic veins connect with the heart by way of the ductus 

 Cuvieri. 



The splanchnic veins of the digestiv : tract and its appendages 

 are primarily related with the intra-embryonic portions of the 

 great veins of the yolk-sac, the omphalomesenteric veins. We 

 have already seen how the proximal ends of these veins unite 

 to form the sinus venosus; they continue to fuse posterior to the 

 sinus venosus, and form thus the ductus venosus, around which, 

 as we have seen, the liver develops. The ductus venosus and 

 the ductus Cuvieri are the only vessels emptying directly into 

 the heart, until the time when the pulmonary veins appear. 

 The omphalomesenteric veins, entering the embryo, pass across 

 the body cavity to the mid-line, beneath the gut and between 

 the two liver diverticula. 



It should be noted here that between each omphalomesenteric 

 vein and the dorsal body wall an extensive fusion occurs, form- 

 ing an incomplete oblique partition through that part of the 

 body cavity immediately posterior to the heart. These fusions 

 are known as the lateral mesocardia, and they are of considerable 

 importance in the later history of the cavities of the body (Fig. 

 139). The ductus Cuvieri pass from the dorsal body wall to 

 the sinus venosus through the anterior parts of the lateral 

 mesocardia. 



Posterior to the ductus venosus the two omphalomesenteric 



