400 
DR. M. TONGE ON THE DEVELOPMENT OF THE SEMILUNAR 
rounded ridge on the left side of the termination of the truncus arteriosus forms the 
principal part of the septum between them. The semilunar valves of each artery are 
larger and more developed, and the sinuses of Valsalva are beginning to appear; but 
there is still no pocketing of the outer valves. They are shown in Plate XXXII. fig. 32. 
About the end of the eighth day, at the 189th hour of incubation, the aperture in the 
infundibular septum is a very minute slit, and very soon afterwards is quite closed up. 
The anterior and inner valves of each vessel are more membranous, with a solid pyra- 
midal corpus arantii ; the lateral valves are just beginning to pocket. The valves have 
gradually become semilunar in shape when viewed from below. 
The latest period at which I have been able to observe the development of the valves 
is the thirteenth day of incubation. They are then miniatures of those of the heart at 
full term, though rather thicker in comparison with their area. This is still more the 
case a few days earlier. Hence the principal changes in them after the closure of the 
infundibular aperture seems to be an increase in area and gradual diminution in thick- 
ness, so that, concurrently with the increase in size and strength of the other parts of the 
heart, they gradually assume the membranous form characteristic of the perfect valves. 
The great vessels are now quite separated down to their roots, and the heart itself presents 
externally very much the appearance of the adult heart. 
Recapitulation. 
I will now recapitulate briefly the principal steps in the formation of the aorta and 
pulmonary artery. 
At first, just before the division commences, the truncus arteriosus, round and smooth 
on the inside, ends abruptly in the three pairs of branchial arteries, third, fourth, and 
fifth, which then exist. It is slightly thickened round its ventricular aperture. 
Then, at the 106th hour of incubation, the division between the fourth and fifth pair 
of branchial arteries extends nearly horizontally into the vessel, as a plane septum with 
forked margin. At the same time the canal of the vessel just below this becomes con- 
stricted by the formation almost simultaneously, 
1°, on its anterior and left surface, of two flattened pyramidal promimences separated 
by a groove — the rudiments of the anterior semilunar valve of each artery ; 
2°, on its posterior and right surface of a flattened ridge, afterwards becoming promi- 
nent and pyramidal, and extending gradually down the posterior surface of the vessel. 
As these growths enlarge, the forked septum grows downwards into the artery, twisting 
from left to right, its left leg passing between and separating the anterior prominences, 
and its right leg passing along the central portion of the posterior ridge. 
By the time the forked septum has reached the part where the anterior valve-rudi- 
ments first appeared, these are more distinctly developed, and the rudiments of the inner 
semilunar valves of each artery have begun to grow out from the posterior ridge on 
each side of the septum, an unoccupied space being left on the wall of the vessel on each 
side for the development of the outer valve in each artery. 
