DEVELOPMENT OF ATRIAL SEPTl'M-PIG EMBRYO 'MV.^ 



and more until it is lost uniong the trabeculae of tlie lateral 

 wall of the atrium. The right sinus valve does not seem to 

 take any part in its formation, though both Rose and Tandler 

 ('13) consider that the upper part of this valve is involved. 



The final disposition of the various parts of the septal and 

 valvular apparatus is shown in figure 9 which represents a dis- 

 section of the heart of a foetus of about 85 mm. Owing to the 

 preponderating growth of the upper part of the atria, the region 

 of the septa appears sunken into the entrance of the inferior 

 cava, while the orifice of the superior cava (V.c.s.) remains high 

 up on the posterior wall. Septum I, now the valvula foraminis 

 ovalis (V.f.o.) lies in the bottom of a slight depression or fossa 

 whose margin is formed for the most part by the limbus fossae 

 ovaHs (L.V.). The free, fimbriated border of the valvula 

 extends obliquely into the cavity of the left atrium and overlaps 

 the limbus on that side. The oblique cleft between them is the 

 foramen ovale. Above the fossa and extending downward on 

 each side of it, the Umbus is thick and muscular. This part 

 is formed by septum II. 



Continuous with the anterior end of septum II is the left 

 sinus valve {V.v.s.) which in this place is muscular like the 

 septum and may be said to form part of the limbus. When 

 traced backward and upward around the fossa, the valve be- 

 comes reduced to a thin, pale streak which finally crosses the 

 posterior part of the Hmbus proper and then extends along the 

 left margin of superior caval orifice where it gradually fades out. 

 This is practically in agreement with the descriptions given by 

 Born, Rose and others. 



The right sinus valve {V.v.d.) is still well marked at this 

 stage. Its lower anterior part bounds the orifice of the inferior 

 cava laterally and extending forward and medially partially 

 covers the orifice of the coronary sinus. The first portion rep- 

 resents the vah^la Eustachii, the last {X in fig. 9) the valvula 

 Thebesii of other mammals. In the pig the right sinus valve 

 never becomes divided structurally. There is always a nar- 

 row cleft between it and the sinus septum {S.sn.). The upper 

 part of the right valve extends as a thin narrow membrane 



