728 THE CIRCULATORY SYSTEM 



eventually proceed latero-ventrad in their development along the outer lateral 

 aspect of the somatopleural mesoderm to the caudal regions of the forming 

 heart, where they turn ventrad along the outer aspect of the somatopleural 

 layer of the hypomere. In the region where the anterior cardinal veins turn 

 ventrad toward the heart region, each anterior cardinal vein is joined by a 

 posterior cardinal vein. The latter proceeds forward from the posterior end 

 of the developing embryo, lying along the outer aspect of the nephrotomic 

 portion of the hypomere below the primitive epidermal tube (fig. 332B). 

 The union of the anterior and posterior cardinal veins on either side forms 

 the common cardinal vein. The latter travels postero-ventrally along the outer 

 aspect of the somatopleure until it reaches the upper limits of the caudal 

 region or sinus venosus of the developing heart. In this area, the splanchno- 

 pleural layer (epimyocardium) and the endocardial layer of the developing 

 sinus venosus, bulge laterad to fuse with the somatopleural layer of the 

 hypomere. This area of contact between the epimyocardial layer of the sinus 

 venosus and somatopleural mesoderm produces a bridge across the coelomic 

 space. The two posterior, dorso-lateral regions of the sinus venosus thus ex- 

 tend dorso-laterad on either side across the coelomic space to join the somato- 

 pleure. Each common cardinal vein perforates through the somatopleure in 

 this area and empties into the sinus venosus at a point lateral to the entry of 

 the two vitelline veins (fig. 332C). This bridge established across the coelomic 

 cavity from the somatopleure of the body wall to the splanchnopleure of the 

 heart forms a lateral mesocardium on either side. The two lateral mesocardia 



Fig. 332. Early development of primitive vascular system including tubular heart. 

 (The diagrams included in this figure should be studied together with descriptions in 

 Chapter 10 relative to tubulation of the major organ-forming areas of the early embryo.) 

 (A) Diagram of the early bilaterally developed vascular tubes (capillaries) which form 

 in relation to the primitive gut tube. This system of capillaries constitutes the first or 

 early vitelline system of developing circulatory structures. (B) The cardinal or primary 

 venous system is added to the primitive vitelline system. (C) The area of union between 

 the early vitelline and cardinal systems at the caudal end of the heart. (D) The basic 

 (fundamental) condition of the vascular system. (E) Two diagrams showing the union 

 of the vitelline and cardinal systems distally between the somites and near the nerve cord. 

 The three vascular tubules to the left in this drawing show an early relationship of the 

 intersegmental arteries and veins, and the drawing of the three vascular tubules to the 

 right depict a later stage of this developmental relationship. (F-M) Stages in the de- 

 velopment of the early tubular heart in shark, frog, and chick embryos. As the mammal 

 is similar to the chick it is not included. (F-H) Early development of the heart in 

 Squalus acanthias. (F) The lower, mesial edges of the hypomeric mesoderm begins to 

 cup around the primitive subintestinal capillaries. (G) Later stage. (H) A transverse 

 section through the heart which is now in the form of a straight tube comparable to 

 that shown in Fig. 339A. (I-K) Early stages in the development of the frog heart. 

 Observe that the ventral areas of the two hypomeres become confluent and later form 

 a trough-like cup around the forming subintestinal capillaries below the foregut. (Redrawn 

 from Kellicott, 1913. Outlines of Chordate Development. Henry Holt, N. Y.) (L-M) 

 Early development of the chick heart. (L) At about 26 hrs. of incubation. (M) About 

 30 hrs. of incubation. 



