PBOCEEDINGS OF SECTION D. 511 



left subclavian. From the abdominal aorta arises a single hypogastric 

 artery. The most remarkable variations in the great vessels of the 

 right twin are, therefore — 



1. The junction of the aorta and pulmonary artery. 



2. The abnormal origins of the right and left pulmonary arteries. 



3. The presence of but one hypogastric artery. 



Venous Abnormalities : Both Tivins. — The superior venae cavae are 

 represented by a single trunk developed mainly in connection with the 

 left twin, and joined by a smaller vessel from the right twin. The 

 single trunk so formed opens into the right auricle of the heart of the 

 left twin. 



The inferior venae cavse are two in number — one from each twin. 

 They ascend from their respective abdominal cavities through the liver, 

 in which they fuse together to form a single trunk, which opens into the 

 inverted right auricle of the right twun. 



Such being the more remarkable of the variations found in this 

 specimen, it is now necessary to say something as to their embryo- 

 logical explanation. 



Origin of Double Terala. — Amongst the numerous theories which 

 have been put forward to account for the origin of double terata, one 

 of the earliest was that which regarded them as due to the fusion of 

 tw^o originally separate ova. This theory is not, however, supported 

 by fact, and is untenable. A second theory of diplogenesis supposes 

 the splitting or fissuring of an originally single embryo ; but here, again, 

 neither experimental teratogeny nor observed facts support such a view. 

 Neither of these theories being sufficient to explain the numerous 

 phenomena recorded, " it is now reasonable," according to Ballantyne, 

 (2) '' to suppose that the double nature of the organism is determined 

 before the appearance of the rudiments of the embryo in the germinal 

 area .... and that the formation of double monsters is decided 

 either before or during impregnation, and not after the appearance of 

 the embryo. It is due, therefore, to a germinal cause." 



Working on this hypothesis. Berry first endeavored to explain his 

 two cases of human thoracopagi (1) by assuming the formation of two 

 primitive grooves on a single ovum produced by bilateral segmentation. 

 The position was, however, untenable, and was abandoned in favor of 

 the supposition that " the two embryos had been developed from a 

 single ovum without bilateral segmentation." This latter theory was 

 found to account for the various abnormalities then recorded, and it is 

 perhaps not unreasonable to assume that this may be found to be the 

 correct theory of the development of thoracopagous monsters. 



In favor of this theory of the evolution of two primitive grooves 

 upon a single ovum without bilateral segmentation, and the consequent 

 production of some form of double terata, there are the following 

 facts : — 



1. Such a condition has been seen and described. References to 

 two such cases will suffice. Bryce (3) has described a case of anterior 

 duplicity in a chick embryo of 34 hours in which there were two separate 

 notochords throughout. The medullary canals were in part separate 



