20 ABNORMAL CONDITIONS OF THE AXIS. 



except that the two faces are not perfectly symmetrical, 

 the mouth of one being a small circular opening. 



Mus. Brit. 



97. Twin foetal Hares united by the anterior extremities of the 



axes. The anterior portion of each axis has been arrested 

 in development, so that the faces exhibit the characters 

 of epencephalic deficiency, cyclopia, agnathia, fusion of 

 the ears, &c. Presented by Sir W. Blizard, 1811. 



98. Twin foetal Hares in a similar condition. There is a frontal 



proboscis on one face. 



B. Opisthozygosis*. 



Syn. Hypogastrodidymus, Gurlt. 

 Ischiopagus, Geoff. 



In Opisthozygosis two distinct axes are united at their posterior 

 extremities, or at some point near their posterior extremities. The 

 embryos remain completely separate above the umbilicus, but 

 unite below by the visceral laminae. One foetus is generally very 

 defective. Sometimes the axes are bent at right angles, in opposite 

 directions, at the point of union. Opisthozygosis is very rare in 

 animals, and the pelvic bones and posterior limbs only of the 

 second axis are usually developed in them, as in No. 103. A 

 similar condition in birds is seen in Nos. 45 and 46. 



99. Human female twins united by Opisthozygosis about the 



sixth month of gestation. The viscera had been re- 

 moved and the spinal columns and pelvis much damaged 

 by a hasty examination. The viscera were fortunately 

 preserved, and are seen in No. 100, 



The sacrum of one side only has been developed. It is 

 situated at right angles to the two spinal columns, which 

 are placed end to end, the bodies of the vertebrae being 

 united without any perceptible break. The sacrum is 

 entirely cartilaginous, no traces of the second sacrum 

 exist. The pelves are otherwise perfect ; but the right in- 

 nominate bone of one foetus is united with the left in- 

 nominate bone of the other, and vice versa. The ilia are 

 united to the lumbar vertebras by elongated transverse 



behind, wy6a> to unite. 



