TERATOGENESIS. 611 



probably belong certain cases of partial duplicity in the pelvic region, with 

 sometimes an extra set of genitalia. Possibly also a few cases of a third lower 

 extremity would come under this head. 



Multiplicity. Monochorionic triplets are rare, only a few cases being 

 recorded. Two cases of monochorionic quadruplets are on record, and 

 one case of quintuplets. Incomplete multiplicities are extremely rare. One 

 case of incomplete triplicity has been described tricephalus. Two verte- 

 bral columns were present in this monster, bearing one and two heads re- 

 spectively. Two thoracic cavities, each enclosing a heart, were separated by 

 a thin septum. The abdominal viscera were single. The lower half of the 

 body and the lower 'extremities were normal, as were also the genital organs, 

 which were male. 



ORIGIN OF SYMMETRICAL DUPLICITY. 



The origin of duplicities has always been most difficult to explain, and 

 the many solutions suggested have been replete with conjecture. The diffi- 

 culty has been caused by the lack of direct observation upon formative stages 

 either in the lower or higher animals. Within recent years, however, experi- 

 mental work upon the lower forms has begun to throw some light upon this 

 obscure problem. Among the theories which have been formulated are two 

 that stand out most clearly the fusion theory (Marchand, Ziegler) and the 

 fission theory (Ahlf eld and others) . 



According to the fusion theory, there are present two originally distinct 

 anlagen within a single ovum. These two anlagen may develop separately and 

 independently and produce twins. They may come in contact during develop- 

 ment and fuse to a greater or lesser degree, thus producing some kind of dupli- 

 cate monster. If fusion does occur it occurs between similar parts of the two 

 anlagen; in other words, like tissues and organs fuse liver with liver, muscle 

 with muscle, bone with bone, and so on. Such unions, however, probably 

 occur only in very early stages of development, for when tissues are once formed, 

 union is effected with much greater difficulty. Consequently fusions between 

 two anlagen, leading to double monsters, probably take place at a very early 

 period of intrauterine life. 



According to the fission theory, duplicity is the result of the division of a 

 single anlage in the earliest stages of development, before the formation of the 

 primitive streak. The cleavage is produced by mechanical resistance of the 

 zona pellucida. Since the greatest mass of growing material is in the head 

 region, the resistance is greatest there, and hence it is argued that duplicities 

 would be most common in the head region, which accords with the facts. A 

 modification of the fission theory to explain duplicities which affect a relatively 

 small area has been suggested. Incomplete anterior duplicity, for example, is 



