THE PRIMITIVE BLOOD VASCULAR SYSTEM 259 



The two proximal bulbar swellings fuse and continue the spiral division of the 

 bulb toward the interventricular septum in such a way that the base of the pul- 

 monary trunk, now ventrad and to the right, opens into the right ventricle, while 

 the base of the aorta, now lying to the left and dorsad, opens into the left ven- 

 tricle close to the interventricular foramen, through which the two ventricles 

 still communicate (Fig. 266 B). 



Closure of the Interventricular Foramen. — The interventricular foramen in 

 embryos of IS to 16 mm. is bounded : (1) by the interventricular septum; (2) by 

 the proximal bulbar septum; and (3) by the dorsal portion of the fused endo- 

 cardial cushions (Fig. 266). Soon these structures are approximated and fuse, 

 thereby forming the septum membranaceum, which closes the interventricular 

 foramen. The atria -ventricular valves arise as thickenings of the endocardium 

 and endocardial cushions of the atrio-ventricular foramina (Figs. 260 and 261). 

 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 ven- 

 tricular wall differentiates, the valvular anlages are undermined, leaving their 

 edges attached to the ventricular walls by muscular trabecule, or cords. The 

 muscle tissue of both the valves and trabeculae soon degenerates and is replaced 

 by connective tissue, forming the chordae tendinece 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. 



Differentiation of the Myocardium. — The myocardium, at first uniformly spongy, 

 becomes compact at the periphery. The inner bundles remain trestle-Kke, forming the 

 trabecule carncB and the papillary and moderator muscles around all of which the originally 

 simple endocardial sac is wrapped. The myocardial layers, at first continuous over the sur- 

 face of the heart, become divided by connective tissue at the atrio-ventricular canal, leaving 

 a small bridge alone. This connecting strand, located behind the posterior endocardial 

 cushion, forms the atrio-ventricular bundle. 



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. 



PRIMITIVE BLOOD VASCULAR SYSTEM 

 The first paired vessels of human embryos are formed as longitudinal anas- 

 tomoses of capillary networks which, however, originate first in the angioblast of 

 the yolk sac and chorion. In the Eternod embryo of 1.3 mm., in which the somites 

 are still undeveloped the paired vessels are already formed (cf. Fig. 268). 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 



