THE VASCULAR SYSTEM 



173 



in a cat embryo of about nine pairs of mesodermic somites 

 (Fig. S6a) . These ''lateral hearts " are the continuation forward 

 of the omphalomesenteric veins after they have reached the 

 intraembryonic region. Widely divergent at their caudal ends 

 they converge cranially so as to lie close together for a short 

 distance. The primordia consists essentially of two parts, an 

 inner endothelial layer which eventually becomes the endocardial 

 lining of the adult heart, and a heavier outer part, the myo- 

 epicardial mantle, which forms the muscular walls of the adult 

 organ. In an embryo of about twelve pairs of somites (Fig. 

 86^) the two lateral moities fuse in the midline of the body to 



Aorta 

 Jruncus Anteriosus 



Fretum Halleri 



Ventriculus 

 Bui bus Cordis 

 rium 



Fig. 86c.— Pig Embryo of Twenty Somites. (After Sabbi, Contrihiilions to 

 Embryology, Carnegie Institution of Washington, 226, plate 5.) 



form a simple tube-like heart resembling in many respects 

 the condition found in the earlier fishes. The primitive heart 

 in its further development then bends on itself and twists 

 so as to bring the caudal or venous end forward and dorsal to 

 the cranial or arterial end. This is well shown in Fig. 86c 

 after Miss Sabin's figure of a pig embryo of about twenty somites. 

 Eventually by a fusion of parts and certain other changes the 



