358 C. V. MORRILL 



figure). This upper segment is broad and low where it faces 

 ostium II but becomes narrow and sharp in the posterior wall. 

 Septum I meanwhile has changed its position relative to septum 

 II, so that its free border now faces upward and forward, and 

 ostium II has become an oblique cleft, the definitive foramen 

 ovale, between the septa. 



Before considering the later changes in septum II it will be 

 convenient to compare the conditions found in the pig with 

 those described for other forms. According to Born septum II 

 first appears (rabbit and man) in the upper part of the posterior 

 wall of right atrium a little to the right of septum I. It is a 

 crescentic spur which encroaches upon the spatium interseptale 

 and forms the principal part of the limbus Vieussenii. In the 

 pig irregular muscular ridges develop in the same locality but 

 they vary greatly in form, size and number. In the model of 

 the 15.2 mm. embryo, two such ridges can be distinguished 

 (fig. 6). W^ien septum II has extended backward to the posterior 

 wall, one or more of these ridges is incorporated in it, but no one 

 of them is sufficiently well-marked to be taken as a starting 

 point for the developing septum. Further, at the time when 

 the anlage of septum II is first distinguishable in the lower 

 anterior wall (7.9 mm. embryo) the spatium intersepto-valvu- 

 lare is relatively very wide and numerous small irregular ridges 

 api^ear in its upper posterior wall. Along with the narrowing 

 of the spatium, the ridges decrease in number until finally they 

 are reduced to one or two which, as stated, are incorporated in 

 the septum. 



An examination of several human embryos seemed to indicate 

 that much the same sort of process takes place there. In an 

 embryo of about 12 mm., total length, at least three muscular 

 ridges could be distinguished in the upper, posterior wall of the 

 spatium intersepto-valvulare, while in one of about 14 mm. 

 only one such ridge appeared which, however, was quite promi- 

 nent." In the lower anterior part of the right atrium of both 



■ Through the kindness of Dr. Thyng I have hiid the opportunity of examin- 

 ing the human embryo which formed the subject of his paper ('14). In this 

 specimen (17.8 mm.), the region of the spatium intersepto-valvulare was entirely 

 devoid of muscular ridges. 



