1026 THE VASCULAK SYSTEM. 



which lies along the dorsal wall of the primitive alimentary canal, is the primitive dorsal aorta ; 

 the bend of the hook is the first aortic arch ; the short ventral limb of the hook, which lies 

 in the ventral wall of the fore-gut and the dorsal wall of the pericardium, is the primitive ventral 

 aorta; it is continuous, at the anterior margin of the umbilical orifice, that is at its own 

 caudal end, with the vitelline vein, which is carrying blood from the yolk-sac to the embryo. 



The condition described is that found in embryos about 1'4 mm. long, possessing six meso- 

 dermal somites (Fig. 818). 



In embryos in which the mesodermal somites have increased to fourteen pairs the posterior, 

 or caudal, portions of the primitive ventral aortae have fused together to form a single heart 

 (Fig. 819). 



The Primitive Heart. The primitive heart lies in the dorsal wall of the pericardium and, 

 therefore, in the ventral wall of the fore -gut. As it grows more rapidly in length than the wall 

 to which it is attached, it bends upon its long axis and bulges ventrally into the interior of the 

 pericardium. As it projects into the pericardium it pushes before it the immediate bounding 

 wall of the pericardia! cavity, which thus becomes converted into the visceral pericardium. The 

 visceral pericardium surrounds the heart, and passes from its dorsal border to the ventral wall of 

 the fore-gut as a double layer which constitutes the dorsal mesocardium. 



The portion of each primitive ventral aorta which lies cephalwards of the heart forms the 

 ventral root of the first aortic arch. At this stage the primitive dorsal aortae are still separate 

 from one another, and each gives off a series of dorsal intersegmental branches, and a series of 

 ventral branches which pass across the side walls of the primitive gut on their way to the wall 

 of the yolk-sac. 



The caudally situated ventral vessels, as in the earlier stage, form a plexus on the side walls 



of the primitive gut from 

 Posterior cardinal veins which the umbilical arteries 







When 



mesenteric) When the embryo has 



2nd aortic arches | fcw and P ossesses about twenty- 



ist aortic arches MT\^^^^ / l^^fc^. three mesodermal somites, 



Anterior cardinal veins thT^e To^of ^fusion *a 



Sinus venosus Umbilical , ft a slightly iater stage 



arteries the fusion of the two primi- 



Vena umbiiicalis impar tive dorsal aortae extends 



PIG. 820.-SCHEMA OF VASCULAR SYSTEM OF AN EMBRYO WITH TWENTY- 



THREE SOMITES. (Arteries after Felix, modified.) - . 



where one of the ventral 



branches of each primitive 



vessel becomes enlarged and forms the origin of the stem of the primitive umbilical artery. Still 

 later the comparatively small continuations of the primitive dorsal aortae, which are continued 

 caudalwards from the twenty-third somite to the end of the caudal region, fuse together to form 

 the middle sacral artery the dorsal aorta, as such, terminating at the twenty-third body somite. 



The Primitive Veins. In embryos 1*4 mm. long two primitive veins are present on each 

 side in the body of the embryo the lateral umbilical veins and the vitelline veins. The lateral 

 umbilical veins are the divisions of the vena umbiiicalis impar, which returns blood from the 

 placenta to the embryo ; and the vitelline veins return blood from the yolk-sac (Fig. 819). During 

 the time in which the embryo increases from 1-4 mm. to 2 '6 mm. additional veins appear. As 

 the body and head of the embryo become larger definite venous channels are formed to return 

 blood from them to the heart, and. in association with the more rapid development of the 

 cephalic portion of the embryo the first entirely intra-embryonic veins to appear are the anterior 

 cardinal veins, one on each side, which return blood from the head, and from the cephalic or 

 anterior portion of the body of the embryo. These veins are present in embryos possessing 

 fourteen mesodermal somites, and each terminates in the 'common trunks formed by the union 

 of the vitelline and umbilical veins of the same side (Fig. 819). 



A little later the posterior cardinal veins, one on each side, develop in the caudal or posterior 

 part of the body. They join the anterior cardinal veins at the level of the caudal end of the 

 heart, and the common trunk formed by the union is the duct of Cuvier, which opens directly 

 into the caudal part of the heart. In the meantime the venous trunk, produced by the union of 

 the lateral umbilical with the vitelline vein, and into which the anterior cardinal vein opened, 

 is absorbed into the heart ; therefore, when the ducts of Cuvier are formed, six vessels, three on 

 each side, open into the caudal portion of the heart the vitelline veins, the lateral umbilical 

 veins, and the ducts of Cuvier. Whilst these changes in the veins are taking place, two additional 

 aortic arches are formed, one on each side. They spring from the cephalic extremity of the heart, 

 immediately caudal and somewhat dorsal to the ventral roots of the first arch ; and they terminate, 

 dorsally, in the dorsal aortae (Fig. 820). 



