122 E VOL UTION AND DISEASE. 



From examples in which dichotomy gives rise to a 

 double-headed monster as in the snake (fig. 67) or the 

 foal, to more complete forms, such as Ritta-Christina, 

 the Two-headed Nightingale, or the sharks (fig. 66), 

 we pass on to instances in which the bond of union is 

 merely a narrow fleshy band, as in the Siamese Twins. 

 From these it is but a step to the origin of separate 

 twin-foetuses by dichotomy of a single ovum. In all 

 cases of duplex monsters which have 

 come under my notice, the individuals 

 composing a double monster were of 

 the same sex, and there is good 

 ground for the belief that when twins 

 are of the same sex and enclosed in 

 the same membranes they are the 

 product of a single ovum. 



Up to this point we have been con- 

 sidering duplex forms in which the 

 body on opposite sides of the cleft 

 equal each other in development. 



67. I he cephalic 



extremity of a double- We are now in a position to consider 

 headed Snake. specimens in which the dichotomy is 



unequal, or if equal at the outset one half grows at a 

 less rate or becomes in great part suppressed. Such a 

 case is shown in the calf (fig. 68). Here we have attached 

 to the sternum of the healthy calf the headless trunk 

 and limbs of a second calf. To such specimens the 

 term parasitic fcetus is usually applied, whilst the normal 

 calf is called the autosite. 



Several specimens of this kind have been investigated 

 in the human subject. A well-known case is that of 



