A HUMAN EMBRYO OF TWENTY-FOUR PAIRS OF SOMITES. 149 



The primary excretory duct occupies a position just dorsal to the nephrogenic 

 cord. In its course caudally it lies close to the ectoderm. Felix was unable to 

 determine whether it developed from the ectoderm or whether it arose independ- 

 ently in the mesoderm, but he doubts that the ectoderm has any participation in 

 its development. Watt 48 also was unable to determine definitely its mode of 

 formation. The caudal end of the primary excretory duct, the point at which 

 its formation is supposed to be taking place, is in my embryo cut sagittally. It 

 is, however, indefinite. Just before its termination the duct is seen lying close to 

 the ectoderm in the mesenchyma. The mesenchyma possesses several mitotic 

 figures in the region in which it terminates. While I am inclined to favor Felix's 

 view of a mesenchymal origin, the evidence found is not convincing. 



VASCULAR SYSTEM. 

 HEART. 



The heart lies in that portion of the body-cavity which is bounded by the 

 pharynx above, the fore-gut behind, the anterior body-wall in front, and the trans- 

 verse septum below. It is still a simple tube and viewed from in front is roughly 

 the shape of the letter U . It is placed so that the loop of the U is directed toward 

 and lies in the right side of the pericardial cavity. The limbs of the U are turned 

 to the left, but upon reaching the body-wall of the left side turn dorsalward at an 

 angle of almost 90 degrees. The upper limb then bends sharply cephalad, joins 

 immediately the pericardial wall, and passes into the ventral aorta. The lower 

 limb bends medially and joins the pericardial wall as it passes into the sinus 

 venosus. The ends of the heart-tube are therefore fixed to the body-wall, but 

 the remainder of the heart lies free within the cavity. 



An examination of the heart-tube shows that it is not everywhere of the same 

 caliber, but presents certain expanded portions separated by more or less definite 

 constrictions. Beginning with the venous end of the heart, the sinus venosus passes 

 into the atrium with but a slight constriction. Figure 2, plate 1, shows that portion 

 of the heart which I regard as the atrium. As seen from behind (plate 3, fig. 1), 

 it presents a V-shaped bend, the apex of which is pointed toward the left, the limbs 

 lying in a horizontal plane. On its upper surface is a distinct irregular projection, 

 as to the significance of which I am in doubt. 



The atrial portion of the heart passes into the ventricular portion with only 

 a slight constriction, the atrio-ventricular canal. The ventricle is much enlarged, 

 having a transverse diameter which is greater than that of any other portion of 

 the heart. It fills up the entire lower right-hand portion of the pericardial cavity. 

 Its cephalic end is bounded by a shallow constriction, in front of which is the 

 bulbus cordis. 



The bulbus cordis is large where it is attached to the ventricle, but gradually 

 becomes narrower towards its cephalic end. It is directed downwards and 

 towards the left. As described by Watt and others, it reaches farther cephalad 

 than any other portion of the heart. It becomes continuous with the truncus 



