HUMAN EMBRYO OF THIRTEEN SOMITES 329 



caudal wall of the venous end of the heart (figs. 3, 6, 7). The 

 dorsal wall of the bulbus cordis has a distinct mesentery con- 

 necting it with the dorsal pericardium. Near the point where 

 the atrial canal joins the ventricle the ventricle has a mesentery 

 which joins the pericardium at the place where the mesentery 

 of the bulbus cordis joins it. Caudal to the junction of these 

 two mesenteries there is a small space dorsal to the atrium which 

 is free from mesentery and represents the future transverse sinus 

 of the pericardium. 



From the dorsal wall of the bulbus cordis a tube-like diver- 

 ticulum is present (fig. 5). I have been unable to find any ref- 

 erences in literature to anything similar to this. The tube runs 

 in the mesentery of the bulbus cordis and at its distal end it 

 comes into close proximity to the ventricle. It is probable that 

 this tube represents a vestige of the space between the two 

 laminae in the closing up of the heart tube and the formation of 

 the mesocardium. Two other tubular spaces of a similar appear- 

 ance may be seen in the mesentery. They have no communi- 

 cation with the cavity of the myo-epicardium. I observed a 

 similar diverticulum from the bulbus cordis in a 4.06 mm. embryo 

 belonging to the collection of the Department of Anatomy of 

 Syracuse University. It may be noted that this bulbus cordis 

 diverticulum does not contain any endothelium. The endothe- 

 lial fibrillae, however, appear to extend into it. 



The endothelium in no place approximates the walls of the 

 myo-epicardium. The caliber of the endothelial tube varies in 

 the different chambers of the heart, being quite constant in the 

 bulbus cordis, enlarged in the ventricle, and greatly reduced in 

 the atrial canal. In the atrium it widens out into the right and 

 left lateral expansions of the atrium. At its cephalic end the 

 endocardium is continued by the ventral aorta which immedi- 

 ately divides to form the first pair of aortic arches. At the 

 venous end of the heart the most distal part of the endocardium 

 represents the sinus venosus. There is no constriction between 

 the sinus venosus and the atrial part of the endocardium. The 

 endothelial fibrillae which have been observed by various authors 



