VALVES IN THE HEART OE THE CHICK 
403 
11. The anterior valve-rudiments commence as transverse thickenings of the interior 
of the vessel, sloping off above and below into the general surface of the vessel, and are 
separated by a slight groove. 
12. The inner and outer valves first appear as simple pyramidal thickenings of the 
vascular wall. 
13. All the semilunar valves are solid at first. 
14. The anterior and inner valves consist of one single segment for each valve. 
15. The outer valve is at first a single pyramidal eminence. It may remain single, or 
become deeply notched and develope into two valves or more. 
16. By the time the third valve in each vessel has appeared, the form of the valves 
has become more defined. They then have the shape of a short crystal of triple phos- 
phate its flat surface being attached, its edge projecting into the vessel, and its ends 
sloping off upwards and outwards above and below. The valves are more developed in 
the direction of their length than transversely, and their course down the wall of the 
vessel is parallel to that which the axis of its canal afterwards assumes. 
17. About the 144th hour of incubation they are, though still solid, sufficiently deve- 
loped to close the canal of the vessel pretty completely, and to prevent much reflux of 
blood into its undivided portion. 
18. By this time the valvular function of the two lips of the opening into the ventricle 
has become quite abolished. 
19. The valves are further developed by the hollowing out of the solid pyramid above 
and near the wall of the vessel, while they grow in other directions. 
20. The pocketing of each valve commences in each in the order of its appearance, 
and begins in the anterior and inner valves of each artery about the time that their bases 
have descended to the level of the bases of the ventricles, i e. at the 147th hour of incu- 
bation, and is distinct in these valves at the 165th hour. The pocketing of the outer 
valve is not distinct till much later. About the time that it commences, the valves have 
assumed nearly their final positions with respect to the base of the heart, and the aper- 
ture of communication between the arterial infundibula is nearly closed up. 
21. After the complete separation of the aortic and pulmonary infundibula from each 
other, the further changes in the semilunar valves consist principally in increase in size 
and diminution in thickness, so that they become more and more membranous, 'pari 
passu , with the growth of the other parts of the heart. 
22. The aperture in the septum of the ventricles does not close up entirely, in the 
manner commonly supposed, but finally developes into the aortic infundibulum. After 
its closure has proceeded to a certain extent, the lower half of the right ventricular 
border of the aperture unites with the lower border of the septum between the aorta 
and pulmonary artery in a peculiar manner, described more at length above. This sepa- 
rates the pulmonary infundibulum and the root of the pulmonary artery from the root 
of the aorta and the canal of the aperture in the septum, which then forms the aortic 
infundibulum. 
3 H 2 
