1868.] 



Valves in the Heart of the Chick. 



333 



termination of the original right leg of the fork in the central part of the 

 left-hand lip of the ventricular slit, while the posterior portion passes off 

 slantingly upwards and forwards as a ridge, which forms 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 arterial 

 infundibula, by whose closure the pulmonary infundibulum and root of the 

 pulmonary artery become separated from the root of the aorta and the 

 canal of the aperture in the septum, which then becomes the aortic infun- 

 dibulum. This process is completed about the end of the eighth day. 

 The separation of the vessels does not become visible externally till it has 

 advanced a considerable distance down the truncus arteriosus, and the 

 semilunar valves are considerably developed. 



The division of the truncus arteriosus into the aorta and pulmonary 

 artery does not therefore take place by the formation of two oppositely 

 situated longitudinal ridges, and their subsequent growth together, but 

 occurs, as above described, by the extension into it of a plane septum from 

 between the fourth and fifth pair of branchial arteries, and which twists 

 down the vessel along the line of a single thick pyramidal ridge which 

 forms gradually on its posterior aspect. 



The formation of the semilunar valves is very closely connected with the 

 process of division of the truncus arteriosus, and the following are the new 

 facts arrived at with respect to their origin and development : — 



1 . It is a remarkable fact that the rudiments of the semilunar valves 

 first appear on the interior of the truncus arteriosus at a considerable 

 distance from the heart, near the termination of the truncus arteriosus in 

 the branchial arteries, and not near the heart, as one might have been led 

 to expect. 



2. It is also very remarkable that the rudiments of the anterior and 

 inner semilunar valves of each artery make their appearance before the 

 partition, which has already begun to separate the aorta from the pulmonary 

 artery, has quite descended to that part of the truncus arteriosus in which 

 these valves originate. 



3. The rudiments of the anterior semilunar valves of the aorta and 

 pulmonary artery are the first to appear, those of the inner valve of 

 each artery the next, and those of the outer valves the last. The deve- 

 lopment of the last valve to appear remains behind that of the others 

 throughout. 



4. The anterior valve-rudiments appear close together, rather on the 

 right side of the anterior surface of the truncus arteriosus, about the 106th 

 hour of incubation, simultaneously with the commencement of the division, 

 and a short distance below it, and opposite the commencement of the 

 ridge which forms on the posterior surface of the vessel, and which appears 

 about the same time. 



5. The rudiment of the inner valve of each artery grows from the 



