DEVELOPMENT OF ATRIAL SEPTUM-PK; EMBRYO 359 



these embn'os there was a thick coiiiu'ctive tissue ridge or promi- 

 nence continuous with tlie fused endocardial cushions and 

 partly overlaid and invaded by developing muscle. As in the 

 pig this musculature was continuous with that of septum I 

 and the sinus valves. The resemblance between this structure 

 and that described for the pig in the corresponding place is so 

 close that I am inclined to think it represents the origin of 

 septum II in man. Indeed Thyng ('14) in his description of a 

 human embryo notes the presence of a ''ridge or tubercle" in 

 the same region and states that "from the relation which the 

 tubercle bears to the septum primum and the left sinus valve 

 it can scarcely be doubted that it would eventually form part 

 of the adult limbus fossae ovalis." 



Born seems to have overlooked the early appearnace of this 

 ridge entirely, though he states that in man the bay of the 

 crescent (septum II) swings further downward in later stages 

 and finally unites with the lower end of the left sinus valve. 

 He thus includes the region in which the ridge is found in the 

 pig. In the rabbit, however, septum II remains as a narrow 

 short crescent in the anterior upper wall of the atrium. One 

 may conclude from this that the ridge if present at all is poorly 

 developed in this animal. 



Rose ('89) in summarizing the results of his earlier work ('88) 

 speaks of septum II (limbus or annulus ovalis) as follows: 



On the anterior and upper atrial wall there appears a ridge-like 

 infolding, the septum musculare which, together with the septum inter- 

 medium is formed into a closed ring diaphragm, the 



annulus ovalis The septum intermedium arises from a 



connective tissue spina vestibuli overlaid, however, on its upper surface 

 by a thin continuous muscle layer, which unites with the broad atrio- 

 \entricular cushions, the latter fused together in the middle. 



It is apparent that this description corresponds very closely 

 with that given for septum II in the pig with the exception that 

 the upper part of this septum corresponding to Rose's septum 

 musculare is not formed by an infolding of the atrial wall 

 but by a local thickening. In his later paper ('89) following 

 Born's work. Rose modified his description of the atrial septa 



