ANATOMY OF A 17.8 MM. HUMAN EMBRYO 71 



A large portion of the right sinus valve subsequently disappears 

 but its caudal part will persist to form the valvulae venae cavae 

 inferioris and sinus coronarii. On the right of the atrio-ventric- 

 ular orifice a portion of the developing tricuspid valve (V.t.) is 

 seen. 



The principal outlet for the right ventricle (Vent.d.) is now 

 the truncus pulmonalis (Tr.puL), although the presence of the 

 small interventricular opening {F.int., pi. 1) still allows some 

 blood to pass out by way of the aorta. The aortic septum {S.) is 

 practically complete at this stage, and is seen in plate 4, separat- 

 ing the root of the aorta from the conus arteriosus iCon.art.) 

 of the right ventricle. 



The left atrium {At.s., pi. 1) is partially separated from its- 

 fellow on the right by the septum primum. A quadrilateral 

 opening in the septum., the ostium secundum (O.s.), places the 

 two atria in direct communication. Leading into the left atrium 

 on its dorsal side two pulmonary veins are shown (Vv.pul.d.). 

 Between the left atrium and ventricle there is a marked protu- 

 berance of the fused endocardinal cushions, which represents in 

 part the anlage of the bicuspid valve. The vessel {V. card. com. s.) 

 seen in section immediately caudad of the left atrium is the part 

 of the left common cardinal which will become the coronary 

 sinus. 



The left ventricle is still in communication with the right 

 by a small opening (F.int.) the foramen interventriculare 

 which is directed obliquely dorso-ventrally from left to right. 

 This eventually becomes closed towards the right ventricle by 

 complete fusion between the aortic and interventricular septa. 

 When this has occurred, the aorta (Tr.aor.) will become the only 

 outlet of the left ventricle. The ventricular wall immediately 

 ventral to the anlage of the tricuspid valve (V.t.) separates the 

 left ventricle from the right atrium, and has been named by 

 Hochstetter ('98) the septum atrio-ventriculare. This septum, 

 together with that formed by the fusion of the aortic (S.) and 

 interventricular septa, form the septum membranaceum ventric- 

 ulorum of the adult heart. 



