226 TEXT-BOOK OF EMBRYOLOGY. 



The further development of the heart consists of various changes in the 

 shape of the tube and in the structure of its walls. At the same time the dila- 

 tation of the ccelom (primitive pericardial cavity) in the cervical region is of im- 

 portance in affording room for the heart to grow. In the chick, for example, 

 the tube begins, toward the end of the first day of incubation, to bend to the 

 right; during the second day it continues to bend and assumes an irregular 

 S-shape. This bending process has not been observed in human embryos, but 

 other Mammals show the same process as the chick. In a human embryo of 

 2.15 mm. the S-shaped heart is present (Fig. 196). The venous end, into 

 which the omphalomesenteric veins open, is situated somewhat to the left, ex- 

 tends cranially a short distance and then passes over into the ventricular portion. 

 The latter turns ventrally and extends obliquely across to the right side, then 

 bends dorsally and cranially to join the aortic bulb which in turn joins the 

 ventral aortic trunk in the medial line. The endothelial tube, which is still 

 separated from the muscular wall by a considerable space, becomes somewhat 



Vent, aortic trunk 



FIG. 198. Ventral view heart of human embryo of 4.2 mm. His. 

 The atria are hidden behind the ventricular portion. 



constricted at its junction with the aortic bulb to form the so-called /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 4.2 mm. lies in the same transverse plane as 

 the ventricular portion. The latter lies transversely across the body (Fig. 198). 

 At the sa*ne time two evaginations appear on the venous end, which repre- 

 sent the anlagen of the atria. In embryos of about 5 mm. further changes 

 have occurred, which are represented in Fig. 199. The two atrial anlagen are 

 larger than in the preceding stage and surround, to a certain extent, the proxi- 

 mal end of the aortic trunk. As they enlarge still more in later stages, they come 

 in contact, 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. 199) ; the right part is the 

 anlage of the right ventricle, the left part, of the left ventricle. At the same time 

 the atrial portion has moved still farther cranially so that it lies to the cranial 



