33^ Dr. Tonge on the Development of the Semilunar [Apr. 30, 



artery, twisting gradually from left to right, its left leg passing between 

 and separating the anterior semilunar valve rudiments, and its right leg 

 growing into the central portion of the oblique ridge on the posterior sur- 

 face, now becoming prominent and pyramidal, and separating from each 

 other the rudiments of the inner valves. Between the outer and inner 

 semilunar valves in each artery there is a vacant space left on the wall of 

 the vessel, from which the outer semilunar valve in each artery afterwards 

 grows out, the outer valves appearing later than the others. The division 

 of the truncus arteriosus proceeds by the gradual growth downwards of the 

 forked septum along the course of the ridge on the posterior surface, which 

 gradually becomes more prominent, the right leg of the fork, which pro- 

 ceeds along it, being always a little in advance of the other. The anterior 

 or left leg of the fork corresponds with the right margin of the anterior 

 aortic valve, and terminates almost immediately on the anterior surface, no 

 ridge being formed along the anterior surface as there is along the pos- 

 terior. As the forked septum between the aorta and pulmonary artery 

 grows diOM^n the vessel, the semilunar valves gradually become more deve- 

 loped, and the rudiments of the outer valves appear. They appear soon 

 after the 1 1 7th hour of incubation, by which time the aorta and pulmonary 

 artery are separated for some little distance, 



"During these changes the aperture into the ventricle has become a rect- 

 angular slit, passing horizontally backwards and to the right, and having a 

 left-hand and a right-hand lip, the left-hand lip sloping from before back- 

 wards and upwards into the artery, and joining the lower end of the ridge 

 that has been gradually forming on the posterior surface of the vessel. 

 As the division proceeds the ends of each lip of the ventricular slit dis- 

 appear, and the central portions, especially of the left-hand lip, become 

 more prominent. By this a channel is left in front and towards the 

 left, and behind and to the right. By the time the division has de- 

 scended to the ventricular aperture, the original right-hand leg of the 

 forked septum has wound round to the centre of the left-hand lip of the 

 slit, the left-hand leg to the centre of the right-hand lip, so that the aortic 

 channel has passed from front to back, and the pulmonary channel from 

 back to front, the anterior portion of the ventricular slit thus becoming 

 the root of the pulmonary artery, and the posterior portion the root of 

 the aorta. 



The septum of the ventricles has been gradually forming during the 

 process of division of the truncus arteriosus, and by the time the division 

 and valves have descended nearly to the base of the ventricles, there re- 

 mains merely an oval aperture in the upper portion uniting the ventricular 

 cavities. It forms a short canal with a left ventricular border and a right 

 ventricular border. The arterial infundibula are finally separated from 

 each other by the union of the lower half of this right ventricular border 

 ■with the lower border of the forked arterial septum. The anterior portion 

 of the right ventricular border is continued upwards and forwards into the 



