402 
DR. M. TONGE ON THE DEVELOPMENT OE THE SEMILUNAR 
pair of branchial arteries, from the branchial arteries towards the heart, the septum 
being at first situated nearly horizontally, and then twisting from left to right as it grows 
down the vessel, and following the line of a single thick rounded and somewhat pyra- 
midal ridge that forms gradually along the posterior surface of the vessel. 
3. The opening from the ventricle into the truncus arteriosus is at first circular, but 
soon becomes a slit with two lips, from a greater development of the vascular tissue 
in this situation. These lips at first exercise a valvular function, in preventing the 
reflux of blood from the truncus arteriosus into the ventricle. As the semilunar valves 
become developed, this valvular function gradually becomes abolished, that of the semi- 
lunar valves being substituted for it. 
4. The valvular function of the lips of the slit becomes abolished by the widening of 
its ends and the greater development of the central portion of each lip, especially of 
the left-hand one. The widened ends of the slit ultimately form the roots of the aorta 
and pulmonary artery, which have then become separated by the growth of the septum 
down the vessel into the central portion of each lip. 
5. It is a remarkable fact that the rudiments of the semilunar valves first appear on 
the interior of the truncus arteriosus at a considerable distance from the heart , near the 
termination of the truncus arteriosus in the branchial arteries, and not near the heart, as 
one might have been led to expect. 
6. It is also very remarkable that the rudiments of the anterior and inner semilunar 
valves of each artery make their appearance before the partition, which has already 
begun to separate the aorta from the pulmonary artery, has quite descended to that part 
of the truncus arteriosus in which these valves originate. 
7. The rudiments of the anterior semilunar valves of the aorta and pulmonary artery 
are the first to appear, those of the inner valve of each artery the next, and those of the 
outer valves the last. The development of the last valve to appear remains behind that 
of the others throughout. 
8. The anterior valve-rudiments appear close together, rather on the right side of the 
anterior surface of the inside of the truncus arteriosus, about the 106th hour of incuba- 
tion, simultaneously with the commencement of the division and a short distance below 
it, and opposite the commencement of the ridge which forms on the posterior surface of 
the vessel and which appears about the same time. 
9. The rudiment of the inner valve of each artery grows from the corresponding side 
of the ridge which forms gradually on the posterior surface of the vessel a little later 
than the anterior valves. 
10. The rudiment of the outer valve in each artery arises from that part of the inside 
of the wall of the truncus arteriosus left vacant, between the outer margins of the rudi- 
ments of the anterior and inner valves, soon after the 117th hour of incubation. It arises 
level with the other valves, when the aorta and pulmonary artery are already separated 
from each other for some little distance, and therefore a little nearer to the heart than 
the other valves, though still at a considerable distance from it. 
