MORPHOGENESIS OF CIRCULATORY SYSTEM 757 



in the human at about the fifth week, and in the pig at about 4 mm. or 17 

 days. This process is similar in the human and the pig, and while the following 

 description pertains particularly to the pig it may be applied readily to the 

 developing human heart. In the pig, as in the chick, a crescentic fold or septum 

 of the atrial chamber begins to grow caudally toward the atrioventricular 

 opening from the antero-dorsal region of the atrium. This septum forms the 

 septum primum or interatrial septum I (fig. 340A). As this septum grows 

 caudad, two thickenings, the endocardial cushions, one dorsal and one ventral, 

 arise in the atrioventricular opening (fig. 340B). The endocardial cushions fuse 

 and divide the atrioventricular canal into two openings. The septum primum 

 ultimately joins and fuses with the endocardial cushions, but the septum as 

 a whole is incomplete, an interatrial opening being present (fig. 340C). Mean- 

 while, the sinus venosus shifts more completely toward the right atrium, and 

 the opening of the sinus into the right atrium also shifts dextrally. This permits 

 an enlarged area to appear between the interatrial septum and the valvulae 

 venosae, or valves of the sinus venosus guarding the sinu-atrial opening. In 

 this area, interatrial septum II or septum secundum, arises as a downgrowth 

 from the atrial roof (fig. 340C, D). This second septum eventually produces 

 a condition as shown in figure 340D. The arrow denotes the passageway or 

 foramen ovale in the septum secundum and also the outlet for the blood into 

 the left atrium over the dorsal part of the valve of the foramen ovale (valvula 

 foraminis ovalis), derived from the atrioventricular end of septum I. This 

 condition persists until birth. The valve of the foramen ovale derived from 

 septum I prevents the backflow of blood from the left atrium into the right 

 atrium. 



The atrioventricular valves are shown also in figure 340D, together with 

 the fibrous attachments of these valves to the muscular columns of the left 

 and right ventricles. The atrioventricular or cuspid valves arise as thickened, 

 shelf-like growths of connective tissue, to which the tendinous cords from the 

 papillary muscles become attached. The left and right ventricles are produced 

 as in the chick by the upgrowth from the ventricular apex of the interventricular 

 septum. In the human, the interventricular septum fuses with the endocardial 

 cushions during the eighth to ninth weeks. The papillary muscles projecting 

 inward into the ventricular cavities (fig. 340D) represent modifications of the 

 trabeculae carneae (fig. 340B). 



3 ) Fate of the Sinus Venosus. The developing superior and inferior venae 

 cavae open into the right horn of the sinus venosus. As the right atrium enlarges 

 it absorbs this right horn mto its walls and the venae cavae obtain separate 

 openings into the right atrium (fig. 340D). The body of the sinus venosus 

 becomes the coronary sinus which opens into the right atrium below the 

 opening of the inferior vena cava. The coronary veins empty into the coronary 

 sinus. The left horn of the sinus venosus may persist as a part of the oblique 

 vein of the left atrium (fig. 340F). 



