SOCIETY OF THE UNIVERSITY OF ABERDEEN. 



73 



separated, and those of the left twin are almost normal. An acquaint- 

 ance with the normal V-shaped bend made under ordinary conditions 

 by the ventricular part of the embryonic tubular heart to the right 

 and a reference to Fig. 5 will explain the order of the ventricles from 

 right to left as previously described. The communication which exists 

 between the two ventricles of the right twin is explained by the 

 non-development of the pars membranacea septi, and the origin of the 



Fig. 5. Diagram to illustrate the development of the ventricles. 



aorta and the pulmonary artery from the one ventricle by the partial 

 failure in the division of the aortic bulb and the imperfect separation 

 of the ventricles. 



The explanation of all the cardiac abnormalities is therefore re- 

 ferable to, first, a fusion in the region of the sinus venosus, and there- 

 after to the imperfect separation of the auricles and ventricles of the 

 right twin, and it is not without interest to note that it is the right 



twin which is mainly at fault. 



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