352 



THE DEVELOP.AIEXT OF THE CHICK 



the pulmonalis, which lies dorsal to the aorta distally, is gradually 

 transposed to its left side. The third division of the aortic- 

 pulmonary septum arises near the opening of the bulbus into 

 the ventricle in the form of two ridges of the endocardium on 

 the right and left sides respectively of the bulbus, the pulmonary 



division lying ventral and the 

 aortic division dorsal to tne 

 incipient partition. A third 

 slight endocardial ridge of the 

 proximal part of the bulbus is 

 described (Hochstetter, Greil) 

 at this stage, but it soon dis- 

 appears. The proximal bulbus 

 ridges may be seen on the fifth 

 day; on the sixth day they are 

 well formed; on the seventh day 

 they have united to form a par- 

 tition which becomes continu- 

 ous with the partition in the 

 distal portion of the bulbus. 



Fig. 201. — A. Section through the 

 truncus arteriosus of an embryo of 5 

 mm. head-length. 

 B. Section through the distal por- 

 tion of the bulbus arteriosus of the 

 same embryo. (After Greil.) 



A., Aorta. P., Pulmonahs. A. S. ao. 

 p., Plane of the septum aortico-pulmo- 

 nale. 1, 2, and 3, Ridges prolonging 

 the septum aortico-pulmonale. 



Thus the separation of the aor- 

 tic and pulmonary trunk is completed down to the ventricle. 



The semilunar valves arise by excavation of three endocar- 

 dial thickenings in each trunk formed at the caudal end of the 

 distal division of the bulbus (Hochstetter, Greil). The origin 

 of these thickenings is as follows. Both the aortic and pulmonary 

 trunks receive one each of the original endocardial ridges of the 

 distal portion of the bulbus owing to the course of the aortic- 

 pulmonary septum. Each also receives half of the ridge along 

 which the septum of the truncus is prolonged. A third ridge 

 arises subsequently in each between these two. A cavity then 

 arises in each ridge and opens distally into the aorta and pul- 

 monary artery respectively, thus forming pockets open in front. 

 These valves are fully formed at eight days. 



The aortic-pulmonary septum becomes thick early in its 

 historv and the muscular lavers of the vascular trunks, which 

 at first form a common sheath for both, gradually constrict mto 

 the septum, and, separate when the constriction brings them 

 together, so that each vessel obtains an independent muscular 

 wall. Subsequently, a constriction extends from the outer layer 



