DUPLICITY. 25 



perfect twin similarly attached. In this preparation the 

 parasitic limbs are fused as far as the metatarsus. A 

 comparison of these limbs- with the dichotomous limb of 

 a monkey, No. 307, shows the difference between malfor- 

 mation from fission and fusion in a marked manner. 



Hunterian. 

 C. Parazygosis*. 



In Parazygosis twins are united, by the visceral laminse of the 

 trunk, above the umbilicus. They are usually placed face to 

 face, and have the laminse on each side equally developed ; rarely 

 they are placed side by side, and have the laminae of the adjacent 

 sides very deficient. Parazygosis is extremely rare in animals, 

 although it is the commonest form of duplicity in the human 



foetus. 



a. Xiphopagus, Geoff. 



The least degree of parazygosis. The union is by the xiphoid 

 cartilages of the sterna and by the abdominal walls, above the 

 umbilicus. 



105. A wax model of the band of union of the " Siamese Twins " 



made in 1830, when they were 19 years of age. In child- 

 hood the band was proportionately much larger and shorter, 

 so that the twins were much closer together. The xiphoid 

 cartilages of the sterna pass across the upper edge of the 

 band. The condition of these twins at the age of 19 and 

 their history is given in the Phil. Trans. 1830, p. 117. 



Presented by G. B. Bolton, Esq., 1830. 



b. Thomcopagus, Forster. 

 Syn. Sternopagus, Geoff. 



In this form the thoracic cavities are united by the visceral 

 laminse ; the foetuses are placed symmetrically face to face. 



106. Female Human thoracopagous twins, 3 inches long (about 



the third month of gestation) . 



Presented by Sir E. Home, Bart., 1804. 



107. A similar twin-birth, 3 inches long (about the third month 



of gestation) . 



The relation of the foetuses was disturbed when this specimen 

 was first put into spirit, so that it docs not apparently conform with 



by the side of, ?vy6w to unite. 



