748 THE CIRCULATORY SYSTEM 



b. The Basic Histological Structure of the Primitive Embryonic Heart 



Structurally, the embryonic heart is composed of two parts. An inner 

 delicate lining, the rudiment of the endocardium, forms as a result of the 

 fusion of the vitelline blood capillaries in the immediate area of the forming 

 heart. The endocardium thus is composed of endothelium (fig. 332F-M). Sur- 

 rounding the endocardial rudiment, there is the epimyocardium derived from 

 the ventro-mesial portions of the hypomeric (splanchnopleural) mesoderm 

 which extends ventrally from the foregut in this area (fig. 332F-M). Basi- 

 cally, the mesial walls of the two hypomeric areas of the mesoderm which 

 lie below the foregut in this region constitute the ventral mesentery of the 

 primitive gut. Consequently, the epimyocardium of the primitive heart may 

 be regarded as modified ventral mesentery. That portion of the ventral mesen- 

 tery which is dorsal to the forming heart forms the dorsal mesocardium, 

 while that part which extends ventrally below the heart forms the ventral 

 mesocardium. The latter is a transient structure, no sooner formed than 

 obliterated in most instances. The dorsal mesocardium tends to persist for a 

 time, more in some species than in others. Caudally, the posterior lateral 

 areas of the sinus venosus project the splanchnopleural mesoderm laterally 

 to contact the lateral somatopleural mesoderm with which the splanchnopleural 

 mesoderm fuses. This outward extension of the caudo-lateral edges of the 

 sinus venosus produces a bridge across the coelomic space from the lateral 

 body wall to the sinus venosus. These bridges on either side across the 

 primitive coelom form the lateral mesocardia. Through these mesocardial 

 bridges, the common cardinal veins empty their contents into the heart. 



Fig. 338. Changes in the converging veins of the heart in the mammalian embryo. 

 (Redrawn and modified from Patten, 1946, Human Embryology, Blakiston, Philadelphia, 

 after McClure and Butler.) (A-F) Developmental changes in converging veins of the 

 human heart. Primitive converging veins of the heart shown in black; hepatic segment 

 of inferior vena cava shown in white with coarse stipple; subcardinal veins shown in 

 light stipple; supracardinal veins in white with crossed lines. (Note: the author assumes 

 the responsibility for adding a vitelline venous segment to the anterior end of the devel- 

 oping inferior vena cava. As a result of observations on developing pig, cat, and opossum 

 embryos, the author is convinced that a vitelline segment is contributed to the developing 

 posterior vena cava in the mammal.) (A) Primitive basic condition. (B-F) Later 

 stages as indicated. (F) Adult condition. The following contributions appear to enter 

 into the formation of the inferior vena cava, viz., (I) a very short vitelline segment; 

 (2) an hepatic segment; (3) an anastomosis between the hepatic segment and the sub- 

 cardinal interrenal anastomosis; (4) a subcardinal-supracardinal anastomosis; (5) a 

 right supracardinal segment caudal to the kidneys; and (6) a posterior cardinal contri- 

 bution in the pelvic area. Note also that the uzygos vein is formed from the anterior 

 end of the right posterior cardinal vein plus the right supracardinal with its connections 

 with the hemiazygos vein. Observe further that the superior vena cava is composed of 

 the right common cardinal vein from the area of juncture with the azygos vein to the 

 point of its entrance into the right atrium. (G-J) Formation of the hepatic portal vein 

 in the pig. (Redrawn and slightly modified from Patten, 1948. Embryology of the Pig, 

 Blakiston, Philadelphia. 



