598 TEXT-BOOK OF EMBRYOLOGY. 



fus } in which the attachment may extend into the neck and head region, so 

 that there is a union from the head to the umbilicus. This type is distinguished 

 from the anterior union in that the head portions of the twins are united 

 laterally, so that both more or less completely developed faces are turned 

 toward the common ventral side, while the bodies have their ventral sides 

 directed toward each other. 



Anterior Union. In this type of union the attachment may be dorsal and 

 confined to the head (dorsal anterior union), or ventral and reaching as far as 

 the umbilicus (ventral anterior union). 



In dorsal union the heads of the twins are joined at the crowns, so that 

 the two bodies lie in a straight line, or form an angle with each other. Such 

 a monstrosity is known as craniopagus (cephalopagus) . The attachment 

 usually involves the cranial vault, the two brains remaining separated by 

 their membranes within a common cranial cavity. Such monsters are rare 

 and survive but a short time. A very rare variety of craniopagus is the 

 form known as craniopagus parasiticus, in which one twin is reduced to a 

 rudimentary structure and is parasitic upon the other. In all the above cases 

 the term autosite is applied to the better developed twin. 



In ventral and ventro-lateral union the attachment involves the head, 

 neck and thorax syncephalus, cephalothoracopagus janiceps. The twins pass 

 through their development in common, each individual contributing its 

 quota of structure to the composite monster. The sternum is single, the oeso- 

 phagus single, the larynx and trachea double or single, the stomach single, the 

 intestine double. The two hearts may be united, but more commonly are 

 separated, one being situated ventrally, the other dorsally. Two faces are 

 formed, one belonging to each embryo. The faces may be alike or nearly so 

 (Janus symmetros), or one may be misplaced or unequally developed (Janus 

 asymmetros), which often results in cyclopia, synotia, or obliteration of the 

 opening of the mouth. 



In some cases the greater part of the body is single and only a part is double 

 (incomplete duplicity). The malformation may affect only the upper end or 

 head (superior incomplete duplicity), or only the lower end (inferior incomplete 

 duplicity). In the former case the skull is single, with possible traces of a 

 double formation diprosopus. There are two faces with varying degrees of 

 fusion between them; all four eyes may be present, or the two approximated 

 eyes may be fused or they may be wanting (diprosopus tetroph-, trioph-, 

 diophthalmus). The two mouths may be fused (diprosopus monostomus), 

 and with a greater degree of fusion between the faces the two approximated 

 ears may also be fused or be entirely lacking. In dicephalus the head is 

 double, and sometimes the upper end of the vertebral column. 



Inferior incomplete duplicity is rare. To this category of duplicate monsters 



