VI 



HEART OF BIRDS 



385 



left ventricle. This division comes about in a somewhat complicated 

 tasnion the chief points in which, judging from Greil and Hoeh- 

 stetters descriptions and figures, appear to be as follows. The 

 ventricular portion of the cardiac tube is at an early stage encroached 

 upon by the deep (« bulbo-auricular ") fold which separates the conus 



ats. 



Fig. 183. — Stages in the development of the heart of the Fowl, viewed from the right 

 side. The right wall of the heart has been removed in each ease. (After original 

 drawings by Greil.) 



A, B, proximal ends of ridges of conns ; at.s, atrial septum ; b.a.r, bulbo-auricular ridge ; c, conus 

 arteriosus ; i.v.f, interventricular foramen ; i.v.s, trabecular portion of ventricular septum ; l.a, left 

 auricle ; p, pulmonary artery ; r.a, right auricle ; r.e.c, cut surface of endocardiac tissue of atrio- 

 ventricular opening ; r. V, cavity of right ventricle ; s t systemic aorta ; V, ventricle ; 1 , 2, 3, conus ridges. 



from the atrial part of the heart (Fig. 182, A, b.a.f). The encroach- 

 ment of this fold gives the ventricular part of the tube a squat 

 U -shape. As the ventricle dilates the extent of the encroachment 

 becomes reduced and the fold may now be called the bulbo-auricular 

 ridge of the heart-lining (Fig. 183, A, b.a.r). During the fourth 

 day this becomes extended tailwards along the ventral wall of the 

 vol. II 2 c 



