362 C. V. MORRILL 



right sinus valve, where it blends with the floor of atrium. 

 Owing to the extreme height of the right valve the inferior vena 

 cava and coronary sinus appear to open into a common chamber 

 flanked by the valve. 



The further changes in the right atrium may now be consid- 

 ered. From the 21 mm. stage (figs. 7 and 8) up to about 58 

 mm. the relations of the parts in question change very little. 

 Septum I or the valvula foraminis ovalis, as it may now be 

 called, becomes thinner and somewhat folded. Its free border 

 is fimbriated. The left sinus valve is still well developed, its 

 lower end blending with the root of septum 11. It now lies so 

 close to the septal wall of the atrium that the spatium inter- 

 septo-valvulare is reduced to a narrow cleft. The right sinus 

 valve remains high, guarding the orifices of the inferior cava 

 and coronary sinus. The sinus septum between the two orifices 

 is behind and medial to the valve but not fused with it. In 

 two specimens (58 to 62 mm.) and possibly a third, a new open- 

 ing appeared in the lower anterior part of the right sinus valve 

 close to its attached border. The course of the blood-stream 

 from the coronary sinus in this case was directly into the lower 

 anterior part of the atrial cavity rather than upward, back- 

 ward and medially toward the foramen ovale. The usual path- 

 way, however, was not shut off. His ('85) described a secondary- 

 opening of the coronary sinus in human embryos and assumed 

 that it represented the normal condition. Other writers have 

 not found this new opening, Born expressly denying its exist- 

 ence. It probably occurs exceptionally in man as in the pig 

 and may represent a premature degeneration of this part of the 

 valve. 



The tensor valvulae (septum spurium) in the older foetuses 

 becomes flattened down considerably in its upper part, though 

 still fairly well marked near the sinus valves. Meanwhile a 

 new muscular ridge, the crista terminal] s has appeared in the 

 atrial wall just to the right of the right valve. Its upper part 

 blends with the trabeculae in the roof of the atrium near the 

 tensor valvulae. Traced downward it passes first close to the 

 attached border of the valve and then diverges from it more 



