THE DEVELOPMENT OF THE VASCULAR SYSTEM 231 



line. The endothelial tube, which is still separated from the muscular wall 

 by a considerable space, becomes somewhat constricted at its junction with 

 the aortic bulb to form the so-called f return Halleri. During these changes 

 the heart as a whole increases in diameter, especially the ventricular portion. 

 Gradually the venous end of the heart moves cranially and in embryos of 



Vent, aortic tmnlr 



FIG. 207. Ventral view heart of human embryo of 4.2 mm. His. 

 The atria are hidden behind the ventricular portion. 



4.2 mm. lies in the same transverse plane as the ventricular portion. The 

 latter lies transversely across the body (Fig. 207). At the same time two 

 evaginations appear on the venous end, which represent the anlagen of the 

 atria. In embryos of about 5 mm. further changes have occurred, which are 

 represented in Fig. 208. The two atrial anlagen are larger than in the 



Right atrium [M .: JH "^KSfc* Left atrium 



Right ventricle '({.' Left ventricle 



Interventricular furrow 

 FIG. 208. Ventral view of heart of human embryo of 5 mm. His. 



preceding stage and surround, to a certain extent, the proximal end of the 

 aortic trunk. As they enlarge still more in later stages, they come in con- 

 tact, their medial walls almost entirely disappear, and they form a single 

 chamber. The ventricular portion of the heart becomes separated into a 

 right and a left part by the interventricular furrow (Fig. 208) ; the right part 



