596 TEXT-BOOK OF EMBRYOLOGY. 



i. Acardiacicompleti. The general development of the acardiacus depends upon 

 the sufficiency of the blood supply which it receives. If there is a well developed 

 anastomosis between the two placental circulations the weaker embryo may 

 receive a moderately good blood supply and develop into a fairly normal foetus. 

 A well formed trunk and head may be present and the extremities may be 

 represented in part or in full. 2. Acardiaci acormi. These may possess only a 

 head, or they may possess a head and traces of a trunk and extremities. 

 Their evolution depends upon unusual combinations of anastomoses in their 

 venous channels. 3. Acardiaci acephali. No head is present. The lower 

 part of the trunk is present, and sometimes other parts of the trunk. The ex- 

 tremities may be complete, incomplete or absent. These forms are also due 

 to peculiar combinations of vascular anastomoses. 4. Acardiaci amor phi. As 

 the name indicates, there is no typical form for the affected embryo. It bears no 

 resemblance to a normal embryo, but is merely an irregular mass of tissue. 



In symmetrical duplicity, instead of the two embryos in one chorion being 

 distinct and separate individuals, they may be joined together to a greater or 

 lesser extent. The two individuals may develop to practically the same 

 degree (equal united twins,) or one may remain more or less rudimentary 

 (unequal united twins.) As may be seen by reference to the scheme on page 605, 

 there are three modes of union posterior, middle and anterior. 



Posterior Union. This may be either dorsal or ventral. In dorsal posterior 

 union the two bodies are joined together in the pelvic region, with the dorsal 

 surfaces of the twins directed toward each other. The umbilicus is double and 

 the two umbilical cords converge toward a common placenta pygopagus. 

 The general anatomical features are as follows. There is a single coccyx 

 and a single sacrum; pelvic bones and symphyses are present in normal 

 number; near the ends of the large intestines the two digestive tubes unite to 

 form a common lumen, and the two recta open through a common anus between 

 the more dorsally situated pair of extremities; the two spinal cords unite near 

 their lower ends to form a single conus and filum terminale; the urogenital 

 tracts are united only to a slight degree. This form of monster is of interest 

 because it is able to live for years; indeed a number of them have lived to 

 maturity. 



In case of ventral posterior union the attachment may be confined to the 

 pelvic region or may involve the entire trunk. In the former instance 

 ischiopagus the right pubic bone of one pelvis fuses with the left pubic bone 

 of the other pelvis, the ventral surfaces of the two sacra facing each other. 

 The axes of the bodies may be in line, or they may form an angle. The con- 

 tinuous ventral surfaces of the two bodies contain a single umbilicus. The 

 organs in the pelvic region may be single or double. The lower extremities 

 may be fully developed, or there may be only three, or rarely two. Sometimes 



