PRIMITIVE BLOOD VASCULAR SYSTEM 



267 



velopment of the septum membranaceum the interventricular foramen is closed. 

 The atrio-ventricular valves arise as thickenings of the endocardium and endo- 

 cardial cushions of the atrio-ventricular foramina. Three such thickenings are 

 formed on the right, two on the left. The anlages of the valves are at first thick 

 and project into the ventricles. Later, as the ventricular wall differentiates, the 

 valvular anlages are undermined, leaving their edges attached to the ventricular 

 walls by muscular trabeculae or cords. The muscle tissue of both the valves and 

 trabeculae soon degenerates and is replaced by connective tissue, forming the chor- 

 dae tendineae of the adult valves. Thus there are developed the three cusps of 

 the tricuspid valve between the right chambers of the heart, and the two flaps of 

 the bicuspid or mitral valve between the left atrium and left ventricle. 



PRIMITIVE BLOOD VASCULAR SYSTEM 



It is assumed that the first paired vessels of human embryos are formed as 

 longitudinal anastomoses of capillary networks which originate first in the angio- 



Dorsal segmenial arfenes 

 Umbilical veins 



Descending aortae 



Umbilical arteries 



Body-stalk 

 Umbilical vein 



VJtelline arte 



fl-imlfivt 



aortic arch 

 imitive heart ' 



Vttello-umbilical frunK 

 Viielline i/eiiu 



Yolk-sac 



FIG. 261. Diagram, lateral view, of the primitive blood-vessels in embryos of 1.5 to 2 mm. (adapted 



from Felix). 



blast of the yolk-sac and chorion. In an embryo of 1.3 mm. in which the somites 

 were not yet developed (Fig. 261) the paired vessels are already formed. They 

 are the umbilical veins which emerge from the chorion, fuse in the body-stalk, then, 

 separating, course in the somatopleure to the paired tubular heart. From the 

 heart tube paired vessels as ventral aorta extend cephalad, then bend dorsad as 

 the first aortic arches and extend caudad as the dorsal or descending aortce. These 

 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. 262). From the yolk-sac numerous veins converge cephalad and form 



