VALVES IN THE HEAKT OF THE CHICK. 
393 
lunar valves of the aorta and pulmonary artery grow out on each side of this pyramidal 
ridge. Its position and relations to the anterior valve-rudiments are shown in Plate 
XXXI. figs. 9 & 10, and also a little later, at the 115th hour of incubation in Plate 
XXXI. fig. 12. In consequence of these formations in the interior of the truncus arteri- 
osus, its canal at the level of the upper margin of the rudimentary valves becomes irre- 
gularly slit-shaped, as in Plate XXXI. fig. 12, the long axis of the slit being inclined from 
the right downwards and to the left across the vessel. As the development of this 
portion of the truncus arteriosus proceeds, the part situated between it and the heart, 
and which is at this time free from ridges on the interior, seems to become somewhat 
shorter and wider. 
About the same time, at the middle of the fifth day, or 108th hour of incubation, the 
opening into the ventricle is a somewhat rectangular slit having a direction nearly hori- 
zontally backwards (Plate XXXI. fig. 11) and a little from left to right. The left- 
hand lip of the slit is more prominent than the right-hand lip, the latter rather nearer 
the ventricle. A small triangular projection fills up each end of the slit. The lips of 
the slit are not quite parallel to each other, as the left-hand lip has an upward slope 
from before backwards. Thus the aortic and pulmonary streams of blood are already 
in process of separation. 
At the next stage of development that I have been able to observe, at the 117th hour 
of incubation (four days twenty-one hours), the ventricle exhibits a very slight division 
by the still quite rudimentary septum of the ventricles. The ventricular end of the 
truncus arteriosus is still slit-shaped, the left lip of the slit running obliquely upwards 
and backwards, while the right is more horizontally situated and less prominent. Above 
this the canal of the truncus arteriosus is rather oval, the anterior surface of its interior 
being smooth and concave, without any appearance of ridges, while its posterior surface 
is already becoming convex by the formation of the lower part of the rounded ridge, 
which terminates by passing obliquely into the left lip of the arterio-ventricular slit, and 
which forms in the lower part of the vessel when scarcely any trace of it is to be seen in 
the central portion. The extension of the partition between the fourth and fifth pair 
of branchial arches has advanced still further down the vessel, so that the trunks of the 
rudimentary aorta and pulmonary artery, when filled with clot, are now distinctly seen 
by transmitted light (Plate XXXI. fig. 13), though there is not at present any indication 
of the division on the external surface of the vessel ; owing to the advance of the outer 
semilunar valves of the rudimentary trunks, and the still slight development of the 
rudiments of the anterior and inner valves, the blood can still pass freely backwards 
from the aorta and pulmonary artery into the still undivided part of the truncus arte- 
riosus, so that, as is seen in the drawing (Plate XXXI. fig. 13), the clot in the latter vessel 
is continuous with those in the former. As soon as the outer (lateral) valve-rudiments 
appear, by which time the anterior and inner rudiments have become still more deve- 
loped, the clots in the aorta and pulmonary artery become cut off from the clot in the 
lower part of the truncus arteriosus, which clot is then small, though above the valves 
mdccclxix. 3 o 
