334 



On the Development of the Semilunar Valves. [Apr. 30, 



corresponding side of the ridge which forms gradually on the posterior 

 surface of the vessel, a little later than the anterior valves. 



6. 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 rudiments 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. 



7. 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 shght groove. 



8. The inner and outer valves first appear as simple pyramidal thicken- 

 ings of the vascular wall. 



9. All the semilunar valves are solid at first. 



10. The anterior and inner valves consist of one single segment for each 

 valve. 



11. The outer valve is at first a single pyramidal eminence. It may 

 remain single, or become deeply notched and develope into two valves, or 

 even more. 



12. 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 phosphate |^^ , its flat surface being attached, its edge 



projecting into the vessel, and its ends sloping off upwards and outwards 

 above, and downwards and outwards below. The valves are more deve- 

 loped 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. 



13. About the 144th hour of incubation they are (though still solid and 

 at some distance from the heart) sufiiciently developed to close the canal 

 of the vessel pretty completely, and to prevent much reflux of blood into 

 its undivided portion. 



14. By this time the valvular function of the two lips of the opening into 

 the ventricle has become abolished. 



15. 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. 



16. 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 incubation, and is distinct in these 

 valves at the 165th hour. The pocketing of the outer valves is not 

 distinct till much later. About the time that it commences, the valves 



