258 THE DEVELOPMENT OF THE VASCULAR SYSTEM 



form the thin- walled semilunar valves (Fig. 266). The anlages of these 

 valves are prominent in embryos of 10 to 15 mm. as plump swellings 

 projecting into the lumina of the aorta and pulmonary artery. 



The two proximal bulbar swellings fuse and continue the spiral divi- 

 sion of the bulb toward the interventricular septum in such a way that the 

 base of the pulmonary trunk, now ventrad and to the right, opens into the 

 right ventricle, while the base of the aorta, now lying to the left and dor- 

 sad, opens into the left ventricle close to the interventricular foramen, 

 through which the two ventricles still communicate (Fig. 267 B). 



Origin of the Right and Left Ventricles. Coincident with the 

 division of the aortic bulb there appears at the base of the primitive 

 ventricular cavity a sagittally placed elevation, the interventricular septum. 

 (Fig. 260 B). It later grows cephalad and dorsad toward the endocardial 

 cushions, and forms an incomplete partition between the right and left 

 ventricles, which still communicate through the persisting interventricular 

 foramen (Fig. 267 B). Corresponding to the internal attachment of the 

 septum there is formed externally the interventricular sulcus (Fig. 267 A); 

 this marks the external line of separation between the large left ventricle 

 and the smaller right ventricle. The interventricular foramen in embryos 

 of 15 to 1 6 mm. is bounded: (i) by the interventricular septum; (2) by 

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

 endocardial cushions (Fig. 267). Soon these structures are approximated 

 and fuse, thereby forming the septum membranaceum, which closes the 

 interventricular foramen. 



The atrio-ventricular valves arise as thickenings of the endocardium 

 and endocardial cushions about 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 ventricular wall differentiates, the valvular 

 anlages are undermined, leaving their edges attached to the ventricular 

 walls by muscular trabeculce, or cords. The muscle tissue of both the 

 valves and trabeculae soon degenerates and is replaced by connective 

 tissue, forming the chorda? tendinecs 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-like, forming the 

 trabeculae carnce 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 

 surface 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 endo- 

 cardial cushion, forms the atrio-ventricular bundle. 



