VALVES IN THE HEART OE THE CHICK. 
401 
During these changes the aperture into the ventricle has become a slit with two lips, 
a left-hand and a right-hand lip, the left-hand lip sloping from before backwards and 
upwards into the artery. The central portion of the vessel remains smooth and free from 
ridges on its inside, but along the posterior surface, above and near the ventricle, the 
pyramidal ridge forms, which ultimately joins the left-hand lip of the ventricular slit. 
As the forked septum grows down the vessel, the semilunar valves gradually become 
more developed, and the outer semilunar valve in each artery appears. The leg of the 
fork which proceeds along the posterior ridge is always prominently developed, while 
that on the anterior aspect of the vessel is but slightly marked. 
By the time the division has reached the ventricular aperture, the original right-hand 
leg of the fork has wound round to the centre of the left-hand lip of the slit, the left leg 
to the centre of the right-hand lip, so that the aortic channel has passed from front to 
back, the pulmonary channel from back to front. The way the septum twists down the 
vessel is shown in the diagram of imaginary cross sections of the vessel just above the 
lower margin of the division at different periods (Plate XXXII. fig 39). 
During the division of the vessel the ends of each lip of its ventricular orifice have 
gradually disappeared so as to leave the central portions prominent, and a channel in 
front and to the left, and behind and to the right. These channels become the roots of 
the aorta and pulmonary artery, by the growth of the forked septum into the prominent 
central portions that remain. 
The arterial infundibula are finally separated from each other by the union of the 
lower border of the forked arterial septum with the portion of the right ventricular 
border of the orifice in the septum of the ventricles, viz. the lower half of this border. 
Its anterior portion is continued upwards and forwards into the termination of the ori- 
ginal right leg of the fork, in the central part of the left-hand lip of the ventricular slit, 
while its posterior portion passes off slantingly upwards into the termination of the 
original left leg of the fork in the central part of the right-hand lip of the slit. Thus 
a twisted hourglass-shaped aperture connects the two arterial infundibula, which 
become separated by its gradual closure. 
The left ventricular opening of the orifice in the septum, with the upper part of the 
right ventricular opening, and the channel between them, remain permanently open, 
and develope into the aortic infundibulum. 
Conclusions. 
The following are the most important new facts at which I have arrived : — 
1. That in the heart of the common fowl the division of the truncus arteriosus into 
the aorta and pulmonary artery does not take place by the formation of two longitudinal 
oppositely situated ridges along the whole length of the vessel and their subsequent 
growth together, as Rathke states to occur in Reptiles, Birds, and Mammalia. 
2. That this process of division of the truncus arteriosus is accurately described as 
occurring by the gradual extension into it of the septum between the fourth and fifth 
mdccclxix. 3 h 
