66 



HUMAN EMBKYOLOGY. 



Yolk-sac artery 

 (later = superior 

 raesenteric) 



2nd aortic arches 

 1st aortic arches 



Anterior cardinal veins 



Sinus venosus 



Umbilical 

 arteries 

 Vena umbilicalis impar 



heart. The more cranially situated parts of the primitive ventral aortse remain 

 separate and take part in the formation of ventral roots of. the aortic arches. 

 Before the single heart is formed other blood-vessels have appeared, which 

 return blood from the chorion and the yolk-sac to the heart. These vessels are 

 the primitive veins. Two veins pass from the chorion into the body-stalk, where 



they fuse together to 



Posterior cardinal veins form the VCIia Umbilicalis 



impar. This divides, at 

 the caudal end of the 

 embryo, into the two 

 lateral umbilical veins, 

 which run to the heart, 

 one along each lateral 

 margin of the embryo. 

 In an embryo 1/3 mm. 

 long (Eternod), in which 

 the paraxial mesoderm 

 had not yet commenced 

 to segment into meso- 

 dermal somites, each 

 lateral umbilical vein 

 received, as it entered 

 the embryo, a large 

 efferent vein from the yolk-sac. This condition, if regular, is very transitory. After 

 a very short time the connexion of the vitelline veins with the caudal ends of the 

 lateral umbilical veins is lost, and the blood is returned from the yolk-sac directly 

 to the heart by two vitelline veins, one on each side, which run along the sides 

 of the vitello-intestinal duct and receive the lateral umbilical veins close to the 

 heart (Fig. 81). 



In the meantime a number of branches have been developed from both the 

 dorsal and the ventral walls of the 7th pair of inter . 

 primitive dorsal aortse; the former segmentai arteries 

 are the somatic pre-segmental and inter- 

 segmental arteries, and the latter are 

 the primitive vitelline arteries. 



In a human embryo which has de- 

 veloped six distinct mesodermal somites 

 the vitelline arteries form a plexus on 

 the sides of the hind-gut area of the 

 wall of the entodermal vesicle, from 

 which the umbilical arteries appear 



FIG. 83. SCHEMA OF VASCULAR SYSTEM OF AN EMBRYO WITH TWENTY- 

 THREE SOMITES. (Arteries after Felix, modified. ) 



Vertebral 

 arteries 



1st pair of inter- 

 segmental arteries 



1st cephalic aortic arch 

 f-ephalic aortic arch 

 3rd cephalic aortic arch 

 4th cephalic aortic arch 

 6th cephalic aortic arch 

 Bulbus cordis 

 Ventricle 



Atrium 



to spring (Felix). The plexus is re- 

 presented in Fig. 81 by the bulbous 

 dilatations. The vessels which enter 

 this plexus arise from the ventral 

 aspects of the primitive dorsal aortse, 

 some distance from their caudal ends. 

 It is probable, however, that the 

 caudal ends of the primitive dorsal 

 aortse are connected with the caudal 

 part of the plexus at the points of 

 origin of the umbilical arteries, though 

 the connexions are not" visible in the sections of the embryo mentioned (Fig. 81). 



Practically the same condition is present in an embryo 1/6 mm. long possessing 

 fourteen distinct somites, except that the main rootlets of the umbilical artery, on 

 each side, are situated farther caudalwards than in the younger embryo, and lie in 

 the region of the most caudal somites (Fig. 82). 



Further Development of the Arterial System. When the embryo possesses 

 twenty-three mesodermal somites, but is still devoid of limbs, the arterial system has 



Sinus venosus 



FIG. 84. DIAGRAM showing stage of five aortic arches. 



