EARLY DEVELOPMENT OF THE HEART AND PAIRED BLOOD VESSELS 257 



the caudal wall of the left atrium at the left of the septum I (Fig. 261 C). This 

 vein bifurcates into right and left pulmonary veins which divide again before 



Pulmonary uiirry 

 Incomplete bulbar seftluin 



Ventral endocardial 

 cushion 



Arrow in pulmonary artery 

 Proximal bulbar septum 



Interventricular, 

 foramen _ 



R. atrio-ventricular . 

 foramen 



R. ventricle 



[trio-ventricular 

 foramen 



. nterventricular 



j septum 



Interventricular sulcus 



Pulmonary artery 

 Base of aorta 



Arrow in aorta 



L. atrio-ventricular 

 foramen 



Interventricular 

 septum 



Fig. 266. — ^Ventral views of stages in the development of the heart to show the differentiation of the 

 bulbus cordis into the aorta and pulmonary trunk (Kollman): A, Heart of a 5 mm. human embryo; B, 

 of a 7.S mm. human embryo. 



entering the lungs. As the atrium grows, the proximal portion of the pulmonary- 

 vein is taken up into the atrial wall. As a result, at first two, then four pul- 

 monary veins open into the left atrium. 

 17 



