VALVES IN THE HEAKT OE THE CHICK. 
397 
described. Where this union fails to occur, a permanent aperture of communication 
remains between the right and left ventricle. 
Later still, at the 144th hour of incubation, or the end of the sixth day, the semilunar 
valves are sufficiently developed to prevent the reflux of blood through them, though 
they are still quite solid. Accordingly we find the aorta and pulmonary artery dis- 
tended by large clots, while the portion of the truncus arteriosus below them is empty 
and collapsed, or only has a small clot extending into it from the ventricle. The trunks, 
however, still exhibit no separation externally, though, as is seen in Plate XXXI. fig. 24, 
their right and left branches are separating. This drawing (the auricles and sides of 
the ventricles having been removed) shows remarkably well the considerable distance 
still separating the arterial aspect of the semilunar valves from the bases of the ventricles, 
owing to the great distance from the heart at which their rudiments first appear. A 
small channel between the edges of the valves is seen leading downwards from either 
vessel, that from the pulmonary artery taking a sweep forwards towards the anterior 
part of the base of the right ventricle, while that from the aorta passes directly down- 
wards towards the oval opening in the interventricular septum. The appearance of the 
semilunar valves, as viewed from above, is seen in Plate XXXI. fig. 25. They are three 
in each artery, are still solid ; their upper surfaces, very slightly concave, are now nearly 
horizontal, and the lumen of the vessel at this point, owing to their close approximation, 
now possesses the form of a three-rayed star. This seems to me to be the stage of their 
development described by Kolliker, though the valves can still scarcely be called cres- 
centic. The edge of each pyramid that projects into the central canal of the vessel is 
prolonged for some way down the wall of the undivided portion of the truncus arteriosus 
towards the ventricles. On looking upwards through the lower end of the section, 
therefore, as shown in Plate XXXI. fig. 26, we see little angular projections of the 
vascular wall into the collapsed canals of the aorta and pulmonary artery, which are 
the lower portions of the rudimentary valves cut across. One of these (inner valve) is 
seen on each side of the section of the posterior ridge. The enormous development 
of this ridge, compared with that of the very slightly marked edge forming the left 
border of the anterior aortic valve, is well seen. The aortic and pulmonary channels 
are seen to communicate below the valves in front of the posterior ridge. 
At the earliest period of the development of the heart the two lips of the opening 
between the ventricle and truncus arteriosus appear to exercise a valvular function, pre- 
venting in a measure the reflux of blood from the truncus arteriosus into the ventricle. 
As the slit widens, the valvular function gradually becomes less and less, and at the 
period of development just described is completely abolished. 
The next stage in the development which it is important to notice is soon after the 
end of the sixth day, at the 147th hour of incubation. The development of the heart 
is now advancing with great rapidity ; its appearance at this time, as seen by transmitted 
light, is shown in Plate XXXI. fig. 27, and Plate XXXII. fig. 28. The breadth of the 
aorta and pulmonary artery and of the valves has increased very much, and the division 
