120 EVOLUTION AND DISEASE. 
the anterior portion of each medullary fold gave rise to 
a head. 
Double embryos vary greatly, according to the degree 
of dichotomy, and the subsequent growth of each half of 
the ovum. The cleavage may affect the cephalic ex- 
tremity only ; this is conveniently called anterior 
dichotomy. Of this the two-headed foal sketched in 
fig. 65 will serve as an example. 
In many cases the cleavage only involves the facial 
portion of the skull, thus producing an animal with two 
tongues and four pair of jaws. The supernumerary 
jaws are, in such cases, conjoined into a single mass 
wedged in between the functional jaws, and not infre- 
quently mistaken for congenital tumours. We shall 
return subsequently to malformations of this class. 
When the cleavage is more extensive than in the case 
of the foal it may give rise not only to two heads and 
necks, but the thoracic region of the body is reduplicated. 
And thus we have a single pair of legs, a common pelvis, 
but the anterior part of the embryo double. Several 
such cases have been recorded in mammals, even in the 
human subject, some of which have been made objects 
of careful physiological study, and subsequently carefully 
anatomized. The most important case of this kind 
was the celebrated Ritta-Christina, born at Sassari, in 
Sardinia, 18209, who after surviving the birth eight months 
and a half, died in Paris. Isidore Geoffroy Saint-Hilaire * 
gives an interesting réswmé of the chief features of this 
case, 
When dichotomy is more complete, two individuals 
* “L’Anomalies des Organisation,” tome iii. p. 119. 
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