493 The Embryology of Hylodes Martinicensis 



hold the same course. As the posterior vena cava, where it joins the 

 heart, acquires greater importance than the vitelline veins, and replaces 

 the posterior cardinals in the trunk, it becomes the principal vein posterior 

 10 the heart. It receives numerous branches from Wolffian bodies, and is 

 the vein of the very vascular tail. 



The hepatic portal vein cannot be distinguished before stage IX. The 

 vitelline vein joins it at the liver, and is its most conspicuous branch 

 until stage XII. 



The anterior abdominal vein has been first observed in stage XIII. It 

 runs in the median line (cf. Fig. M, aav), from the posterior end of the 

 embryo, along the ventral surface of what remains of the yolk, bends 

 back around the anterior limit of the yolk, passes posteriorly for a 

 short distance in the pericardium and enters the liver with the hepatic 

 portal. The stages are wanting to determine whether the anterior ab- 

 dominal has a paired origin, and whether when it first develops, it opens 

 directly into the sinus venosus. In stages XIV and XV, when the 

 yolk has gone from the region of the heart and liver, the anterior ab- 

 dominal passes directly from the ventral wall of the abdomen into the 

 liver, making no loop anteriorly as in stage XIII. The vesical vein, a 

 branch of the anterior abdominal from the bladder, is found in stage 

 XIV. 



Arteries. — The dorsal aorta in stage IV is fully established; it is 

 formed by the union of the aortic arches and extends to the posterior end 

 of the embryo. It might fairly be expected that so large a quantity 

 of yolk as the embryo possesses would be supplied by a fixed arterial 

 system, but it has been impossible to trace any definite arteries to the yolk- 

 mass at this stage. It can only be said that the infolding walls of the 

 posterior gut are surrounded by blood corpuscles, and there is some evi- 

 dence that a branch from the aorta connects with this region. 



In stage VI, a branch of some size passes from the aorta into the 

 mesentery which slings the yolk of the middle region (Fig. F, B3, ma) ; 

 thence it runs posteriorly along the rod of yolk, and posterior gut (Fig. E, 

 ma), and probably gives oif branches on either side, to the yolk. The 

 vessel, reduced in size, persists in later stages as the coeliaco-mesenteric 

 artery (Figs. H8, 0, ma). The point where it leaves the dorsal aorta is 

 slightly posterior to the union of the two aortge, and in the region between 

 the union of the posterior vena cava with the right and with the left pos- 

 terior cardinal. The point of union with the left cardinal is, as it were, 

 thrown back to admit of the passage in the mesentery of this little branch 

 of the aorta. The arterial system in the visceral arches has been worked 

 out as far as possible in the present series ; but it is hoped, as in the case 



