THE VASCULAR SYSTEM 



281 



Soon after the two halves of the heart are united in the mid-ventral 

 line, the heart itself becomes S-shaped as a consequence of its elongation 

 in a confined space. The dorsal curve is posterior and connects directly 

 with the paired viteUine (omphalomesenteric) and umbilical veins. 

 The ventral curve is anterior and extends forward beneath the pharynx 

 as the truncus arteriosus. Circulation has already begun when the heart 



G 



H 



K 



76 HOURS 

 M somites 



100 HOURS 

 45 somites 



Fig. 254. — Ventral views of the heart of chick embryos at successive stages to show- 

 its changes of shape and its regional differentiation. Abbreviations: a. v., constriction 

 between atrium and ventricle; i. v., interventricular groove. (From Patten's "Embryol- 

 ogy of the Chick.") 



is in this tubular condition. By the time the human embryo is two months 

 old, the heart, although its size is minute, has reached its adult form 

 and structure. (Fig. 254) 



The processes involved in converting a tubular heart into a four- 

 chambered one include: i. The increased flexion of the heart so that the 

 posterior atrial portion becomes anterior, while the morphologically 

 anterior ventricular portion hes posteriorly. 2. The formation of a 



