364 C. V. MORRILL 



along the right border of the superior caval orifice ahnost to its 

 upper end. The tensor valvulae has entirely disappeared. 



At this stage the crista terminalis (Cr.ter.) forms a thick ridge 

 extending downward from the roof, in the angle between the 

 posterior and lateral walls of the atrium. It was pointed out 

 that when the crista first appears it is entirely independent of 

 the right sinus valve, but as it becomes broader and more fully 

 developed, the extreme upper part of the valve comes to lie 

 right on its root. There is usually, however, a narrow cleft 

 separating it from the remains of the valve below. For this 

 reason, the crista terminalis of the pig cannot be said to mark 

 exactly the boundary between the primitive sinus cavity and 

 the atrium proper as has been maintained by His, Rose and 

 Tandler for man. 



A glance at figure 9 will show that there is a broad, rounded 

 ridge in the angle between the orifices of the superior and inferior 

 venae cavae (the line marked V.v.s. passes across it). It is 

 formed by a thickening of the musculature of the atrial wall 

 in this region and represents the tuberculum intervenosum 

 (Loweri) of the human heart. There has been some doubt 

 about the occurrence of this structure but recently Tandler ('13) 

 came to the conclusion that it is always present in the adult 

 human heart if hardened in situ, i.e., with the pericardium 

 intact. He suggests the term 'torus Toweri' as more appro- 

 priate. In other mammals he finds it more strongly developed 

 than in man, especially in the horse and still more so in the seal 

 where it forms a 'veritable septum.' In the pig this ridge 

 occurs constantly in older foetal stages. Owing to its position 

 it would tend to direct the blood-stream from the superior cava 

 toward the right atrio-ventricular orifice. 



The chief results of this investigation may be summarized 

 briefly as follows : 



In the pig, the method of formation of septum I and ostia I 

 and II is essentially the same as described in other mammals. 



The present account of septum II, however, differs consid- 

 erably from those of other writers with the exception of Favaro. 

 The anlage of this septum in the pig appears in the lower anterior 



