484 



MISS K. M. PARKER ON THE 



ducts, which ai^e now established as wide vessels, pass across the 

 coelomic cavity to open into the right and left heart-tubes (text- 

 fig. 22). 



Curvature of the median portion of the heart has resulted in 

 the definite establishment of a U-shaped ventricular limb and an 

 auricular portion extending from the left dorsal side of the ven- 

 tricle posterior to its apex and separating, at the opening of the 

 gut, into the right and left halves of the sinus venosus. 



Text-fiL^u^e 22. 





W 



\ 



:i. \ 





Maeropus ruficollis. Transverse section through the Cuvierian ducts. 



A.C.V. Anterior cardinal vein. C.JD. Cuvierian duct. B.A. Dorsal aorta. JSnd, 

 Endothelium, ilf.r. Medullary tube. ilfy. Mj'ocardium. Pr.^. Proamnion. 

 V.C.M. Vena capitis medialis. 



The cephalic portion of the S-shaped heart is somewhat curved, 

 so that, as in the preceding stage, the bulbus (conus) arteriosus 

 lies dorsal to the cephalic extremity of the ventricle. The bulbus 

 arteriosus is continued into a short median ventral aorta which 

 bifurcates to form the first pair of aortic arches. The second and 



