120 E VOL UTION 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, 1829, who after surviving the birth eight months 

 and a half, died in Paris. Isidore GeofTroy Saint-Hilaire 1 

 gives an interesting resume of the chief features of this 

 case. 



When dichotomy is more complete, two individuals 

 1 " L'Anomalies des Organisation," tome iii. p. 119. 



