. The development of the chick; an introduction to embryology. Birds -- Embryology. 352 THE 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 descri
. The development of the chick; an introduction to embryology. Birds -- Embryology. 352 THE 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 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 poc
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