DEVELOPMENT OF ATRIAL SEPTUM-PIG EMBRYO 361 



for the pig. He finds that the niyocardiuiu whicli invades the 

 connective tissue of the septum intermedium becomes more 

 compact and raised up into a distinct prominence on the right 

 side of the insertion of the atrial septum (septum I). This, he 

 says, represents the site of the inferior segment of the Hmbus 

 fossae ovaUs. The prominence is continued upward as a thick- 

 ening of the myocardium of the ventral wall of the atrium on the 

 right of septum I. It corresponds to a broad sulcus externally 

 but there is no infolding as described by Rose and Retzer. In 

 the guinea-pig and sheep, however, Favaro finds that the septum 

 spurium or tensor valvulae bends over the upper wall to join 

 the newly-found Umbus or septum II. This does not occur in 

 the pig as reference to figures 5, 6 and 7 will show. In this 

 form the tensor valvulae^ {T.vv.) extends upward to the roof 

 of the atrium where it is lost sight of among the developing 

 trabeculae in that region. In the human embryos examined 

 the course and relations of the tensor seemed to be more like 

 those in the guinea-pig and sheep as described by Favaro. 



From the first the lower end of the left sinus valve blends 

 with septum II as mentioned previously. The right valve is 

 at fii'st independent of the septum (fig. 5) but later when the 

 septum has increased in height, the valve becomes attached to its 

 right side (figs. 6 and 7). In later stages the right valve in- 

 creases rapidly in height so that when seen from the right side, 

 it completely covers the lower end of the left valve and the root 

 of septum II (fig. 7). When a portion of the right valve is cut 

 away (fig. 8) the connection between the left valve and septum II 

 can be seen. Separating the orifices of the inferior vena cava 

 {V.c.i) and the coronary sinus^ {Snxor.) is a thick ridge, the 

 sinus septum (S.sn.), which extends from the root of septum II 

 downward and to the right toward the attached border of the 



^ The term 'tensor valvulae,' first proposed by Rose ('89), seems more ap- 

 propriate than the older 'septum spurium' of His. This structure probably 

 helps to approximate the valves and render them tense during systole. 



' When the model was sawn through, the cut opened the right extremity of 

 the coronary sinus which forms a deep bay as it turns backward and upward to 

 its orifice (figs. 7 and 8). 



THE AMERICAN- JOURNAL OF ANATOMY, VOL. 20, NO. 3 



