SOCIETY OF THK UNIVERSITY OF ABERDEEN. 69 



from these objections there are others of an even more insuperable 

 character, and consequently this supposition of bilateral segmentation 

 of a single ovum was subsequently abandoned in favour of the follow- 

 ing theory, which appears to offer a more satisfactory explanation of 

 the phenomena, 



1. The Explanation of the Lutenil l<'itxion und the Common Ei>il>lastic 

 CoreriiHj of t lie /-V/.W . I IVHU. This explanation is based on the supposi- 

 tion that the two embryos have been developed from a single ovum 

 without bilateral segmentation. The two primitive grooves were 

 placed side by side in such a way that the caudal ends of the embryos 

 have been rather nearer to each other than the cephalic ends, or at all 

 events the central portions of the primitive grooves have either been 

 nearer each other than any other portions, or have been in actual 

 contact. What I then suppose to have taken place is illustrated in 

 Fig. 2, and granting the original hypothesis two embryos developed 

 from a single ovum without bilateral segmentation an easy explana- 

 tion results as to the lateral fusion of the twins, even to the greater 

 fusion in the second example than the first, and also as to the common 

 epiblastic covering of the fused areas and the single jejunal tube. All 

 are easy of explanation on this hypothesis, and difficult, if not actually 

 impossible, upon any other. 



2. The Explanation of the Single Duodenal and Jejunal Tubes and 

 the Single Liver. The hypothesis adopted in the preceding paragraph 

 assumes the presence of a single yolk-sac common to both embryos, 

 and Fig. 3 illustrates my conception of the evolution of the single 

 duodenal and jejunal tubes from this common yolk-sac. Both the 

 pyloric ends of the duodenal tube have undoubtedly been developed 

 from the independent fore-guts, whilst the intermediate portion, with 

 the biliary duct and the single jejunum have, in my opinion, been 

 developed from the common yolk-sac or mid-gut. I say " in my 

 opinion " because such an assumption necessitates the liver being a 

 mid-gut development, and the numerous works upon embryology which 

 have been consulted appear to me to leave the question an open one. 

 Keith, in his Human Embryology and Morphology, states that the fore- 



