398 
DR. M. TONGE ON THE DEVELOPMENT OE THE SEMILUNAR 
between the vessels has descended so very close to the base of the heart that the undivided 
portion of the truncus arteriosus has disappeared externally, the bases of the semilunar 
valves being now on a level with the bases of the ventricles. The root of the pulmonary 
artery now has a position in front and a little to the left of the root of the aorta, which 
is becoming hidden behind it. There is still, however, no division externally between 
the large trunks. The semilunar valves, both aortic and pulmonary, are still more deve- 
loped in the direction of their depth than transversely; they are still solid. Their 
upper surfaces are horizontal or slightly concave, and hollowed near the arterial wall, as 
is well seen from the shape of the base of the large coloured clot which fills the pulmo- 
nary artery : this shows that the hollowing-out process, by which they become mem- 
branous, is just commencing. The development of the valves proceeds by the gradual 
hollowing out of the solid pyramidal mass of tissue above and next the arterial wall, 
while it grows at the sides and below and towards the middle of the artery. The oval 
aperture of communication between the ventricles is still open ; but its channel is be- 
coming more and more directed towards the aorta, and less towards the right ventricle 
(Plate XXXI. fig. 27). The septum of the ventricles has become much thicker. The 
margin of the left opening of the channel through the septum is becoming rounded off, 
so that the canal is funnel-shaped towards the left ventricle. The lower part of the 
orifice into the right ventricle afterwards closes up, while the upper part leading to the 
aorta, as well as the funnel-shaped canal leading from the left ventricle, remain perma- 
nently open, and become the infundibulum or sinus of the aorta. At the sixth day, and 
earlier, the orifice in the interventricular septum is seen to be formed of a connective 
tissue-like structure, similar to that of the adjacent portion of the truncus arteriosus, and 
quite distinct from the muscular substance of the lower portion of the septum. 
At the 165th hour of incubation the union of the upper part of the ventricular sep- 
tum with the forked lower border of the division between the aorta and pulmonary 
artery, which has now quite reached the base of the ventricles, is approaching completion : 
in other words, the separation of the infundibulum or conus arteriosus of the aorta from 
that of the pulmonary artery is now nearly effected. The drawings (Plate XXXII. figs. 
29, 30, 31) from hearts that had been incubated for this space of time (six days 
twenty-one hours) show the appearance of the right ventricular border of the orifice in 
the interventricular septum, and of the under aspect of the semilunar valves, and of the 
division between the vessels. Figs. 30 & 31 were taken from a heart whose development 
was further advanced than in fig. 29, and show the mode in which the separation between 
the aortic and pulmonary blood becomes completed. 
The lower border of the partition between the aorta and pulmonary artery now appears 
as a thick round ridge on the left side of the termination of the truncus arteriosus, 
arching over above and becoming lost on the interior of the right side of the junction of 
the truncus arteriosus with the right ventricle; its vertical plane is nearly at right 
angles to that of the septum of the ventricles. The pyramidal inner semilunar valves 
are seen projecting from either side of this rounded ridge into the orifices of the aorta 
