122 EVOLUTION 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 
eround 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 
g ROSS body on opposite sides of the cleft 
Wry VS 
iG. 67.—The cephalic : 
extremity of a double: We are now in a position to consider 
headed Snake. 
equal each other in development. 
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 farasitic foetus is usually applied, whilst the normal 
calf is called the azéosite. 
Several specimens of this kind have been investigated 
in the human subject. A well-known case is that of 
