SOCIETY OF THE UNIVERSITY OP ABERDEEN. 77 



in this attempted explanation of the developmental errors responsible 

 for the variations recorded, there seems to be no doubt that the 

 theory upon which this explanation is based is the correct starting- 

 point. That twin embryos do develop side by side upon a single 

 ovum is proved by the remarkable case published by Miss Duncan in 

 the Proceediiit/t* of the Anatomical and Anthropological Society of 

 Aberdeen, 1900-1902, under the title of "The Anatomy of a Double 

 Chick Embryo," and the cases with which this paper deals would 

 further seem to prove that bilateral segmentation of an ovum is 

 unnecessary for the evolution of twins. The condition of the ali- 

 mentary canals proves conclusively that the twins in each case have 

 possessed but one common yolk-sac, bilateral segmentation of a single 

 ovum would result in two yolk-sacs, and hence these cases seem to 

 afford indubitable proof of the truth of the hypothesis adopted herein, 

 namely, double embryos of a single ovum without bilateral segmenta- 

 tion. Further proof of the truth of this theory is afforded by Kamann's 

 " Zwei Falle von Thoracopagus Tetrabrachius," published in the 

 Arc/tiv fur Gynakol., Bel. Ixviii., Ht. 3, where the condition of the 

 alimentary canal in each case was identical with that recorded in the 

 present paper. The present writer's theory, therefore, not only ex- 

 plains the two cases now under discussion, but also suffices for Miss 

 Duncan's case and for those of Kamann. If these several cases and 

 the resulting facts may be accepted as proving the writer's conten- 

 tion, the labour and thought bestowed upon these embryological 

 problems will not have been spent in vain, for two important scientific 

 results follow : 



1. Bilateral segmentation of a single ovum is unnecessary for the 

 production of twins. 



'2. The liver is a mid- gut development. 



