DEVELOPMENT OF ATRIAL SEPTUM-PKi EMBllYO 355 



of the right atiiuni in the angle between the fiooi- and septum 

 I, and just x^osterior to ostium I (0.7). It is continuous with the 

 left sinus valve. Trans^^erse sections through this region (figs. 

 1, 2 and 3) show that at first this spiir (S.II) is composed of 

 connective tissue alone and is sharplj^ marked off, histologically, 

 from the developing musculature of the sinus valves, septum I 

 and the atrial floor. It appears to be nothing more than a pro- 

 jection, toward the right, of the endocardial thickening of sep- 

 tum I (S.I) and is continuous through the latter with the endo- 

 cardial cushions of the atrial canal (En.s. and En.i.). This 

 structure which is the anlage of septum II, is undoubtedly what 

 His ('85) called the spina vestibuli and, taken together with the 

 endocardial cushions, would constitute his septum intermedium. 

 The lower end of the left sinus valve blends wdth it, while the 

 corresponding end of the right valve flattens down in the atrial 

 floor close to it (figs. 2 and 5, Vv.v.). At this stage, the anlage 

 of septum II is not in relation with ostium II. 



In later stages, when ostium I has completely closed, the 

 spur lengthens out into a definite ridge. This is well shown in 

 the model of a 15.2 mm. embryo (fig. 6, S.II). When traced 

 from its point of origin in the anterior inferior corner of the 

 atrium, this ridge extends upwards then bends backward along 

 the roof toward the posterior wall where it flattens down and 

 may become continuous with two or three muscular trabeculae 

 w^hich develop in this region, i.e., the spatium intersepto-valvu- 

 lare. These trabeculae are not constant in number. The por- 

 tion of the ridge in relation to the roof of the atrium^ is broad 

 and low contrasted with its low^er anterior end or root which is 

 sharp and well-defined. Both sinus valves {V.v.s. and V.v.d.) 

 now blend with it, the right, however, slightly covering its 

 lateral side. 



The internal structure of septum II is shown in a transverse 

 section taken at about the middle of its anterior vertical por- 

 tion (fig. 4, S.II). The endocardial thickenings which entered 

 into its formation in an earlier stage, have fused together into 



^ Owing to the position of the model this part of septum II is hidden in 

 figure 6. 



