396 
DE. M. TONGE ON THE DEVELOPMENT OE THE SEMILTJNAK 
ing towards the aorta, at the posterior part of the vessel. The upper surfaces of the 
valves are shown in Plate XXXI. figs. 20 & 23, and the upper surface of the 
anterior valve in each artery is also seen in Plate XXXI. fig. 19, in which figure the 
profile of the inner and outer valves is also seen. The valves are still quite solid. 
Their upper surfaces slope upwards from the centre towards the periphery of the vessel, 
and are slightly concave, especially in the case of the anterior.valves. The inner valve 
of the pulmonary artery and the outer valves of both vessels are seen to be distinctly 
notched ; they do not yet accurately close the canal of the vessel when it is fully dis- 
tended, so that an irregular aperture is seen in the centre. It does not receive exactly 
the form of a three-rayed star till some hours later. 
The present form and position of the lower forked margin of the descending septum 
is seen in Plate XXXI. fig. 21. The plane of the septum passes across the vessel 
from before directly backwards, and the lower forked margin is now very distinctly seen 
to be formed by the union of the prominent posterior pyramidal ridge with the slighter 
anterior ridge bounding the anterior aortic valve on the left, and being almost immedi- 
ately lost on the anterior wall of the vessel. On either side of the fork are seen the 
apertures of the aorta and pulmonary artery, and the pyramidal prominences of the ante- 
rior and inner valves. The under surfaces of the valves are a little more rounded off; 
the outer valves are still not very conspicuous from below. In Plate XXXI. fig. 22 the 
shape of the opening into the ventricle is shown. It is no longer a slit, as its two lips 
have been gradually undergoing the changes by which its anterior portion ultimately 
becomes the upper part of the infundibulum leading into the pulmonary artery, while 
the. posterior portion becomes the upper part of the aortic infundibulum. This is 
effected by a widening of the fissure, principally at its ends, by an absorption or arrest 
of development of the ends of each lip of the fissure, while the development of the cen- 
tral portion proceeds uninterruptedly, and by the hollowing out of the upper surface of 
the front of the posterior lip. A prominence is thus left about its centre, in which the 
posterior pyramidal ridge terminates below. The anterior portion of the forked septum 
ultimately grows into the remains of the central part of the right-hand lip of the fissure. 
During the time that these changes have been going on the septum of the ventricles has 
been gradually growing up between the left and right ventricle, and has now attained a 
considerable development. An oval aperture exists in its upper part, and leads from 
the left into the right ventricle. This aperture has a right border opening into the 
right ventricle, and a left border opening from the left ventricle. The anterior part of 
the right border becomes continuous with the central part of the posterior or left-hand 
lip of the arterio-ventricular opening, and therefore with the lower end of the pyramidal 
ridge on the posterior surface of the vessel. At a later period the posterior part of the 
lower half of the right border of the septal aperture bends round so as to become con- 
tinuous with the central portion of the anterior or right-hand lip of the arterio-ventri- 
cular opening. The bearing this has on the union of the septum between the aorta and 
pulmonary artery with the upper part of the ventricular septum will be presently 
