DEVELOPMENT OF THE HEAHT. 



143 



cavity, where its situation is externally marked by a groove. The free edge of this 

 septum grows forwards and downwards, and the septum (fig. 170, s') gradually 

 separates the auricular cavity into a right and left half, the separation being com- 

 pleted by the junction of its free edge, which shows a distinct endocardial thickening, 



Fig. 173. TWO STAGES IN THE FORMATION OP THE SEPTUM INTERMEDIUM IN THE HEART OP THE HUMAN 



EMBRYO. (His.) 



In A the septum is represented as growing from a triangular area to the left of the sino-auricular 

 orifice in B it has coalesced with the endocardial cushions, and lies like a stopper in the auricular 



r.a, La, right and left auricle ; r.v, l.v, right and left ventricle ; s.r, sinus venosus ; Eu.v, Eustachian 

 valve'; s.sp. septum spurium ; s.s, septum superior ; s.inf, septum inferior ; s.i, septum intermedium ; 

 v.c.s, vena cava superior d extra. 



