64 



HEART AND EMBRYONIC BLOOD-VESSELS 



connected behind with the common stem of the earlier stage in the body-stalk, and 

 bringing blood from the chorion ; and the ducts of Cuvier, formed by the junction of 

 two veins from the body of the embryo (anterior and posterior cardinal). The 

 ducts of Cuvier and allantoic veins effect a junction before they reach the sinus 



venosus. 



The aortic bulb passes into two ventral vessels, which join with the dorsal 

 aortse by two arches. The dorsal aortse in turn sweep back on each side of the 



all 



FIG. 92. PROFILE . VIEW OF A HUMAN EMBRYO OF 

 ABOUT FIFTEEN DAYS, WITH THE ALIMENTARY 

 CANAL IN LONGITUDINAL SECTION. (His.) 



p.v., primitive velum ; end, endothelial tube of 

 heart ; v, yolk-sac ; u.a., umbilical (allantoic) artery ; 

 u.v., umbilical vein ; all, allantoic diverticulum. 



FIG. 91. DIAGRAM OF THE VASCULAR CHANNELS IN A HUMAN EMBRYO OF THE SECOND WEEK. 



(After Eternod.) 



The position of the vessel's will be understood if the diagram be compared with the surface view 

 of the blastoderm at this stage given in fig. 72 (p. 49). The afferent channels (including the 

 two heart-tubes) are coloured blue; the efferent (aortas) red. m.s., m.s., marginal sinus (primitive 

 umbilical veins : the anterior dilated portions of the veins are the primitive heart -tubes, h) ; 

 cs, section of abdominal stalk enclosing all, allantoic diverticulum, a single venous, and two arterial 

 channels ; n.c,, neurenteric canal. The dotted blue lines indicate the position on the back of the 

 yolk-sac, and therefore not seen from this view, of the sinus ensiforme. 



notochord, giving branches (vitelline arteries) to the yolk-sac ; they terminate by 

 bending round the tail end of the embryo into the body-stalk, within which they 

 are carried to the chorion. 



The further changes in the heart and vessels will be treated of in the second 

 part of this volume. It will suffice, at this stage, to have shown that in Primates 

 the embryonic vessels are connected from the earliest period with the chorion, and 

 that by the end of the second week the circulation between that membrane and 

 the embryo is fully established. 





