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MISS K. M. PARKER ON THE 



The curvature, however, does not affect both sides equally, and a 

 marked asymmetry results. The anterior ventricular portions of 

 the heart-tubes are already being pushed backwards so as to lie 

 ventral to the auricular portions. It is clear that by continuation 

 of this curvature with accompanying fusion of the two halves, 

 the typical embryonic relations of auricle and ventricle will 

 ultimately be achieved. 



In the ventricular region of the heart, the right and left endo 

 thelial tubes are approximately equal in size, but where there is 

 an inequality the right is the larger (text-fig. 17, End.) 



Text-figure 17. 



Ferameles ohesula (10.viii.03). 



Transverse section through the ventricular region 

 of the heart. 



A^. Second aortic arch. A.V. Auditory vesicle. B.A. Dorsal aorta. JEnd. Endo- 

 thelium. lf.P. Medullary plate. My. Myocardium. V.G.L. Vena capitis 

 lateralis. V.C.M. Vena capitis medialis. 



In addition to the curvature which is bringing the ventricular 

 region into position ventral to the auricular region, there is a 

 certain amount of 'curvature in the horizontal plane of the 

 embryo. ' ' 



In the right half of the heart, a definite constriction of the 

 endothelial tube marks the limit between the ventricular and 

 auricular portions. On the left side there is no such constriction. 

 Posterior to this constriction the right endothelial tube widens out 

 suddenly, reaching about three times its width in the constricted 

 region. The left endothelial tube in this region widens only very 

 slightly. The right and left heart primordia furthermore show 

 considerable asymmetry as regards curvature, for while the 

 portion of the left tube lying lateral to the open fore-gut is prac- 

 tically straight, the right tube in this region shows well marked 



