392 
DR. M. TONGE ON THE DEVELOPMENT OF THE SEMILUNAR 
pulmonary artery. On looking from front to back (Plate XXXI. fig. 3) or from back 
to front through the septum between them, its cardiac edge is seen to be forked, so 
that it extends further along the sides than the centre of the vessel. Thus, as is well 
seen in the preparation from which the drawing was taken, the rudimentary aortic and 
pulmonary vessels are incompletely separated below, where they first leave the truncus 
arteriosus. That there now exist two channels distinctly separated from one another 
just above the forked septum is shown by a transverse section of the vessel at this point. 
Plate XXXI. fig. 5 is a drawing of such a section, looking from the distal side towards 
the heart. The section made a little lower through the forked part of the septum 
(Plate XXXI. fig. 6) gives the idea of two prominent ridges forming on the interior of 
the vessel, and being about to adhere. They are not, however, continued throughout the 
truncus arteriosus, a section through its central portion being nearly quadrangular and 
free from ridges (Plate XXXI. fig. 7), but at this period almost directly subside into 
the wall of the vessel (Plate XXXI. fig. 8). It is very remarkable that even at this 
early period, when the rudimentary aorta and pulmonary artery have only just appeared, 
the rudiments of some of the semilunar valves are already to be seen a little below the 
forked septum and at a considerable distance from the heart. The situation at which 
they first appear, and their position with reference to the rudimentary vessels, is roughly 
indicated in Plate XXXII. fig. 37. They are pretty accurately represented in the drawing 
(Plate XXXI. fig. 9) of this part of the interior of the truncus arteriosus. On looking 
into the lower end of the section, which has been made just below the rudimentary 
valves, they appear as somewhat flattened pyramidal projections, passing obliquely 
upwards on the wall of the vessel and separated by a triangular groove. These are 
the anterior aortic and pulmonary semilunar valve-rudiments, which are the first to 
appear. On examination by transmitted light their situation is indicated by a slight 
opacity of the vessel, which is constricted at this part. Their upper edges are situ- 
ated just below the termination of the left leg of the fork of the septum, which can be 
seen above them (Plate XXXI. figs. 8 & 9), and which, as the septum passes down 
the vessel, gradually grows between these valve-rudiments, passing from the left side 
rather towards the front of the truncus arteriosus, and separating the anterior aortic 
from the anterior pulmonary semilunar valve. By the time that the partition has 
descended as far as these valves, they have become larger and more developed, and the 
rudiments of the inner semilunar valves have begun to appear. About the time that 
the anterior valve-rudiments appear, the right side and posterior surface of the interior 
of the vessel nearly opposite to them becomes thickened and rather prominent in the 
centre. This is the commencement of a thick pyramidal and somewhat rounded ridge 
that forms gradually along the posterior surface of the vessel. As the septum grows 
down the truncus arteriosus, the right leg of the fork becomes continuous with the 
central portion of this ridge, and follows its line obliquely down the posterior surface of 
the vessel from right to left, the left leg of the fork, as we have seen, passing down on 
the front of the vessel obliquely from left to right. The rudiments of the inner semi- 
