386 BRADLEY M. PATTEN 
of the dorsal mesocardium is attached to the heart. On either 
side it is marked off by a groove extending from the lateral con- 
striction at the point of entrance of the omphalomesenteric vein, 
onto the dorsal surface of the heart (pl. 3, H and I, S-A. c.). 
The dorsal dilation thus bounded may now be differentiated 
definitely from the atrium as the sinus venosus. 
The differentiation of the sinus venosus takes place at the 
same time as the caudal bending of the cardiac loop. It is pos- 
sible that their appearance may be more than casually coincident. 
The caudal bending of the loop causes the blood from the omphalo- 
mesenteric veins to be directed against the dorsal and ceph- 
alic wall of the sino-atrial chamber, rather than toward the 
atrioventricular ostium, as in earlier stages of development 
(pl. 2, H, I, and J). It will be noted that the sinus dilation 
occurs at precisely the point at which the blood current impinges 
against the heart wall. A deduction that the blood current is a 
causal factor in the dilation is alluring, but in default of experi- 
mental evidence, any suggestion to this effect must be considered 
as purely tentative. 
Whatever molding effect the blood stream may exert in the 
process, the demarcation of the sinus venosus becomes more 
and more distinct as the caudal bend in the heart becomes more 
pronounced (pls. 2 and 3, J to L). The caudal bending of the 
loop, too, results in the shifting of the sinus from its original 
position, caudal to the atrium, to the dorsal position it occupies 
at the end of the fourth day of incubation (pls. 2 and 3). At 
this stage the sinus venosus is a pouch-like dilation which receives 
the ducts of Cuvier laterally and the fused omphalomesenteric 
veins caudally. It is marked off from the atrium by a groove, 
which is especially strongly developed caudally and dextrally. 
Already it opens into the atrial chamber somewhat to the right 
of the midline, foreshadowing its later association with the right 
atrium.’ 
2 At this stage the two layers of splanchnic mesoderm which constitute the 
dorsal mesocardium flare out on either side and are reflected over the ducts of 
Cuvier at their points of entrance into the sinus venosus (pl.3, I). These trans- 
verse folds of the mesocardium have been designated (Lillie) as the mesocardia 
lateralia. 
