DEVELOPMENT OF ATRIAL 8El'TUM-PI(i EMBRYO 3o7 



tlio simis valves and scptinu I. Tlie (levelopiiig inuycle elements 

 appear to invade the connecti^'e tissue and gradually to replace 

 it.-* This invasion has not yet begun in the 8.5 mm. embryo but 

 is well advanced in the 12.3 mm. and s'ill more so in the 15.2 

 nmi. described above. The anterior portion of the connective 

 tissue mass or septum intermedium^ (fig. 4 En.c.) broadens out 

 in a lateral direction between the two atrio-ventricular orifices 

 and is joined by the interventricular septum, the line of fusion 

 being toward the right. ^ The fusion is not complete, however^ 

 for the interventricular ostium (fig. 6, O.in.) is still widely open 

 at this stage. If septum II is now traced upward and back- 

 ward into the roof of the atrium, the connective tissue elements 

 gradually disappear and it appears as a low, broad thickening 

 of the atrial musculature which follows the external depression 

 caused by the bulbus in this region. It is not, however, due 

 to an infolding of the atrial w^all, but to a local increase in the 

 developing muscle elements. This upper part of septum II 

 now comes into relation with ostium II (fig. 6, O.II) which 

 meanwhile has become much larger. 



Passing now to the 21 mm. embryo, the further development 

 of septum II (fig. 7, *S.77) is clearly shown. It appears as a 

 crescentic ridge extending from the lower inferior corner of the 

 atrium, upward and backward toward the posterior wall. Its 

 lower segment or root which is thickest and most sharply de- 

 fined is partly hidden by the high right sinus valve iV.v.d.) 

 which blends with its lateral surface. Traced upward it curves 

 over the roof of the atrium bordering ostium II {O.II) and 

 reaching the posterior wall, bends downward somewhat in the 

 region of the spatium intersepto-valvulare (not shown in the 



^ The invasion of the connective tissue mass (septum intermedium) by the 

 musculature of the sinus venosus was described by Retzer ('08) as the source of 

 the Purkinje fibres in the ventricles. The present investigation leads to the 

 conclusion that a part of this muscle, at least, thickens to form the root of 

 septum II. 



'" The term 'septum intermedium' originally used by His ('85), may be con- 

 veniently retained for this structure as suggested by Favaro ('13), although 

 Born and some others discarded it. 



" A portion of the septum intermedium thus separates the right atrium from 

 the left ventricle (conus arteriosus of the aorta) forming the septum atrioven- 

 triculare of Hochstetter. 



