VALVES IN THE HEAKT OF THE CHICK. 
395 
subsequently follows the course of a single prominent pyramidal ridge which forms 
gradually along the whole of the posterior surface of the vessel, being at first a little on 
its right side, and afterwards a little on its left. The septum, therefore, grows down 
the truncus arteriosus in a spiral twisting from left to right. 
The position and shape of the cardiac surfaces of the rudimentary semilunar valves at 
the 130th hour is also shown in Plate XXXI. figs. 16, 17, 18. The anterior valves 
are the most developed, and the outer and inner valves are formed on the same type. 
They are best described as solid pyramidal growths of the vascular tissue, and have very 
much the shape of a short crystal of triple phosphate, the flat base being attached 
to the wall of the artery, while the sloping ends look upwards and downwards, and the 
edge projects into the centre of the vessel. This edge is sometimes a little flattened or 
notched. The upper ends slope upwards and are not at first horizontal, though some- 
times slightly concave. The inner semilunar valve of each artery is seen to have a 
similar form to that of the anterior valve, and to grow out from the corresponding 
convex side of the posterior pyramidal ridge, into which their sloping lower ends are 
continued. The main purpose of this very prominent portion of the posterior ridge 
seems to be to afford a sufficiently large surface on each side for their attachment and 
continuous development, as well as a firm basis of support for them against the pressure 
of the blood-column in the rudimentary trunks. The anterior valves have a sufficiently 
large and firm basis of attachment in the anterior wall of the vessel, and the presence 
of a very prominent ridge between them would interfere with their development and 
narrow the artery unnecessarily. We find, therefore, that they are separated only by a 
very slight edge and a groove. The lateral or outer valves of the aorta and pulmonary 
artery have just appeared. That of the aorta is well seen in Plate XXXI. fig. 18. 
It originates nearly opposite to the other valves at the outer portion of the vessel. It 
has the same pyramidal shape, but is more notched on the inner edge ; and if this notch 
deepens much, the single valve may develope into two valves. The outer valve of the 
pulmonary artery appears about the same time as the outer aortic valve ; it is similar 
to it in shape, and similarly situated with respect to its fellow valves. 
From the foregoing observations it will be seen that Kolliker’s statement, that the 
semilunar valves are at first nothing but horizontally projecting crescentic growths, can- 
not be accepted as quite applicable to them in this early period of their development. 
Ten hours later, at the 140th hour of incubation, we find that the division between 
the aorta and pulmonary artery has extended much nearer the heart, and that the rudi- 
ments of the outer valve in each artery having become more developed, the reflux of 
blood into the still undivided portion of the truncus arteriosus is very inconsiderable. 
Plate XXXI. fig. 19 shows the appearance of this part viewed from behind by trans- 
mitted light. The clots above the semilunar valves have been removed. The channel 
followed by the pulmonary blood is indicated by the direction of the clot in the undi- 
vided portion of the vessel, and passes obliquely upwards across the anterior part of the 
truncus arteriosus. The aortic blood passes directly upwards from the ventricular open- 
3 G 2 
