760 



THE VASCULAR SYSTEMS 



the umbilical arteries to the villi of the chorion. Subsequently, as the nonplacental chorionic 

 villi atrophy, their vessels disappear, and then the umbilical arteries convey the whole of the 

 blood to the placenta, whence it is returned to the heart by the umbilical veins. In this manner 

 the placental circulation is established, and by means of it nutritive materials are absorbed 

 from, and waste products given up to, the maternal blood. 



Forebrain.!--\- 



Aortic bulb... 



Auricle 



Optic reside. 



- Ventricle. 



Omphalo-mesen- 

 teric veins. 



FIG. 535. Head of chick embryo of about thirty-eight hours' incubation, viewed from the ventral surface. 

 X 26. (From Duval's Atlas d'Embryologie.) 



The umbilical veins, like the vitelline, become interrupted by the liver, and the blood returned 

 by them passes through this organ before reaching the heart. Ultimately the right umbilical 

 vein disappears. 



During the occurrence of these changes great alterations take place in the primitive heart and 

 bloodvessels, and now require description. 



Further Development of the Heart. The simple tubular heart, already described, be- 

 comes elongated and bent on itself so as to form an S-shaped loop, the cephalic part bending to 

 the right and the caudal part to the left. The intermediate portion arches transversely from 

 right to left, and then turns sharply forward into the cephalic part of the loop. Slight con- 



Aortic bulb.fmr 



Ventricle. 

 Vena cava superior. 



Umbilical vein.-- i 



^.Maxillary process. 

 .Stomodseum. 

 'Mandibular arch. 



'Auricle. 

 Bile duct. 



FIG. 536. Heart of human embryo of about fifteen days. (Reconstruction by His.) 



strictions make their appearance in the tube and divide it from behind forward into four parts, 

 viz.: (1) The sinus venosus; (2) the primitive auricle; (3) the primitive ventricle; (4) the aortic 

 bulb, which consists of two portions, a proximal muscular portion known as the bulbus cordis, 

 and a distal portion, the primitive aortic stem (Figs. 533 to 535). The constriction between the 

 auricle and ventricle constitutes the auricular canal, and indicates the site of the future auriculo- 

 ventricular valves. 



The sinus venosus is at first situated in the septum transversum behind the common auricle, 

 and is formed by the union of the vitelline veins. The -veins or ducts of Cuvier from the body of 



