DEVELOPMENT OF THE HEART 



257 



proximally; when the bulb in this region is divided by ingrowing con- 

 nective tissue into the aorta and pulmonary artery, the aorta contains the 

 whole of the thickenings b and half of a and c, while the pulmonary 



Pulmonary artery 

 Incomplete bulbar septum 



Aorta 



Dorsal endocardial cushion 



Vtntral endocardial 

 cusiiiun 



Arrow in pulmonary artery 

 Proximal bulbar septum 



Interventricular 

 foramen 



R atrio-ventricular 

 foramen 



R. ventricle 



Atrio-ventricular 

 foramen 



Interventricular 

 septum 



Interventricular sulcui 



Pulmonary artery 

 Base of aorta 



Arrow in aorta 



L. atrio-ventricular 

 foramen 



Interventricular 

 septum 



FIG. 267. Stages of the human heart, in ventral view, to show the division of the bulbus and 

 ventricle: A, 5 mm. embryo; B, 7.5 mm. embryo. 



trunk contains the whole of d and half of a and c (Fig. 266). Distally, 

 the three thickenings now present in each vessel disappear, but prox- 

 imally they enlarge, hollow out on their distal surfaces, and eventually 



17 



