VALVES IN THE HEAET OE THE CHICK. 
399 
and pulmonary artery respectively. The other semilunar valves are seen growing from 
the wall of the vessel. Viewed from below, they still have a pyramidal shape. The left 
ventricular border of the channel through the septum of the ventricles has become more 
sloped off. The right ventricular border is seen in Plate XXXII. fig. 29. It forms an 
oval orifice whose upper half is sloped and rounded off towards the aortic orifice just 
above it, to which the channel through the septum is directed. Its lower half is sharp 
and well defined, and the anterior part of this, near its junction with the upper half, passes 
off obliquely into the rounded ridge on the left side, which forms the greater portion of 
the division between the aorta and pulmonary artery ; this is well seen in Plate XXXII. 
figs. 29, 30, 31. As the completion of the partition proceeds, the posterior part of the 
sharp and well-defined lower margin of the orifice grows off at its junction with the upper 
half sharply upwards and outwards to the right, bending round the upper part of the 
inside of the base of the right ventricle, and then passing forwards to join the right side 
of the division between the vessels, where this loses itself on the wall of the pulmonary 
infundibulum a little below the remains of the central part of the right leg of the arterio- 
ventricular fissure. This process is just commencing in Plate XXXII. fig. 29. In 
Plate XXXII. figs. 30 & 31, it has proceeded to a considerable extent, and the aortic 
and pulmonary infundibula now communicate by an hourglass-shaped aperture 
whose plane is twisted on itself, so that the portion on one side of the constriction 
of the hourglass is nearly at right angles to that on the other. The closure of this 
aperture by the growth of its margin all round completes the separation between the 
aortic and pulmonary infundibula. If we consider it closed by an imaginary plane 
surface, the course of the plane will be at first vertically downwards, then gradually 
twisting from left to right, and at the same time bending towards the left. Plate 
XXXII. fig. 38 further illustrates the mode of completion of the division between the 
two infundibula, which are already beginning to assume their ultimate positions with 
respect to each other. 
The anterior and inner semilunar valves of the aorta and pulmonary artery are just 
beginning to form pockets, the part corresponding to the corpus arantii, and the margin 
of the valve generally, being very thick and solid. The outer valves as yet do not form 
pockets, and their development is less advanced than that of the other valves, as it has 
been throughout. In the preparation from which figs. 30 & 31 were taken, there are 
four valves in the pulmonary artery, the outer valve-rudiment having split into two valves. 
The valves do not appear to descend any more towards the ventricles in their further 
development, but remain nearly stationary in position, the further descent of the parti- 
tion merely completing the arterial infundibula below them. 
The trunks of the aorta and pulmonary artery are now beginning to separate exter- 
nally, and about the middle of the eighth day, at the 180th hour of incubation, the large 
vascular trunks have become quite separate down to the semilunar valves, and have 
attained almost their final relative positions. The aperture of communication between 
the arterial infundibula has closed up till it is now only a semilunar slit, and the thick 
