THE PRIMITIVE BLOOD VASCULAR SYSTEM 259 



Descent of the Heart. At first the heart lies far cephalad in the cervical region, but 

 it gradually recedes during development until it assumes its permanent position in the 

 thorax. 



Anomalies. Dextrocardia is associated with a general transposition of the viscera 

 (p. 195). The aorta and pulmonary artery may also be transposed in the absence of dex- 

 trocardia. Rarely the paired anlages form a double heart. Of the complete or partial 

 defects of the septa, most common is a patent foramen ovale. If this fails to close after 

 birth, the mixed blood produces a purplish hue in the child which is known popularly as a 

 'blue baby. This condition may be persistent in adult life. Incomplete closure occurs in 

 about one in four cases, but actual mingling of the blood is rare, due to an approximation 

 of the overlapping septal folds during atrial systole. 



THE PRIMITIVE BLOOD VASCULAR SYSTEM 



The first paired vessels of human embryos are formed as longitudinal 

 anastomoses of capillary networks, that originate first in the angioblast of 

 the yolk sac and chorion (p. 243). In the Eternod embryo of 1.3 mm., in 

 which the somites are still undeveloped, such paired vessels are already 

 formed (cf. Fig. 268). The umbilical veins emerge from the chorion, 

 fuse in the body stalk, then, separating, course in the somatopleure 



Dorsal inter segmental arteries Descending aorta 

 Umbilical veins 



Umbilical arteries 



Body stalk 

 Umbilical vein 



Primitive aortic arch 

 Primitive heart 

 Vitello-umbilical trunk 



. , _, , - ' Yolk sac 



V i tell me arteries 



FIG. 268. Diagram, in lateral view, of the primitive blood vessels in human embryos of 1.5 



to 2 mm. 



to the paired, tubular heart anlages. From the heart tubes, paired ves- 

 sels, the ventral aortas, extend cephalad, then bend dorsad as the first aortic 

 arches and extend caudad as the descending aorta. These, as the umbilical 

 arteries, bend sharply ventrad into the belly stalk and branch in the wall of 

 the chorion. The chorionic circulation is thus the first to be established. 

 In embryos 2 to 2.5 mm. long (5 to 8 somites), the heart has become 

 a single tube (Fig. 269). From the yolk sac, numerous veins converge 

 cephalad and form a pair of vitelline veins. These join the umbilical veins, 

 and, as the vitello-umbilical trunks, traverse the septum transversum and 

 open into the sinus venosus. The descending aortae give off, dorsally and 

 cranially, several pairs of dorsal inter segmental arteries, and, ventrad and 



