SOCIETY OF THE UNIVERSITY OF AHERDEEN. 67 



also a well-marked fossa ovalis leading into what is functionally the 

 left auricle and which undoubtedly belongs to the heart of the left 

 foetus. The left auricle, much smaller than the right, receives the 

 four pulmonary veins, probably one from each of the four lungs, that 

 is to say, the common pulmonary trunk from each lung has not divided 

 into two, and hence there are only four pulmonary veins instead of 

 eight. Opening out of the left auricle there is the orifice of one of 

 the four auricular appendages and the openings into the right and left 

 ventricles of the heart of the left foetus. 



The ventricles are four in number, their order from right to left 

 is as follows : Right ventricle, left ventricle, right twin ; left ventricle, 

 right ventricle, left twin. The right and left ventricles of the right 

 twin communicate with each other and with the right auricle common 

 to both twins. The aorta and pulmonary artery of the right twin 

 arise from the right ventricle. The right and left ventricles of the 

 left twin communicate with the left auricle common to both twins, 

 but do not communicate with each other. The aorta and pulmonary 

 artery arise normally from their correct ventricles. 



The Developmental Explanation of the Foregoing Variations. The 

 chief variations to be explained may be tabulated as follows : 



A. Common to both cases : 



1. Lateral fusion, more extensive in one case than the other, 



and the common epiblastic covering of the fused areas. 



2. The single duodenal and jejunal tubes and the single liver. 



3. The suppression of the gall-bladder in the one case and its 



presence in the other. 



B. Special to the second case : 



4. The partial fusion of the heart and the peculiar condition 



of the auricles and ventricles. 



5. The condition of the great thoracic vessels. 



Bearing in mind the strong similarity which exists between the two 

 cases as evidenced by the lateral fusion, more extensive in the second 

 example than the first, and the precisely similar nature of the intes- 

 tinal variations, it is fairly obvious that whatever explanation is 



