463 Duplicate Twins and Double Monsters 



another, the details of the palms and soles do not wholly correspond. 

 There seems at present no very satisfactory way of explaining these, 

 unless it be to consider them (1) as true duplicates, and allow the 

 possibility of some lack of correspondence in the friction-skin configura- 

 tion, or (3) as fraternals that chance to look very much alike. Of these 

 alternatives the second is here considered the more probable. 



8. Physical measurements of four sets of duplicate twins between 17 

 and 31 years of age show that they are not identical in size, although 

 the variations are for the most part slight. Differences that depend 

 upon skeletal parts are less than those depending upon the soft parts. 



II. CONCLUSIONS. 



A. On Twins and Their Eelationship to Double Monsters. 



1. Twins belong to two types, duplicate and fraternal. 



3. Fraternal tivins result from the simultaneous ripening and conse- 

 quent fertilization of two separate eggs, and are thus as distinctly differ- 

 ent as are any other two children of different birth. They may be of 

 the same or of different sex ; each develops within a separate chorion and 

 possesses a separate placenta ; they may or may not resemble one another ; 

 the palm, sole and finger markings do not correspond. 



3. Duplicate tivins are the result of the total separation of the first 

 two blastomeres of a single egg, the product of the first cleavage, and 

 therefore possess an identical germ plasm. They are invariably of the 

 same sex; they develop within a common chorion, but possess each a 

 separate umbilical cord attached to a common placenta; they greatly 

 resemble each other, usually to the point of confusion; the palm, sole 

 and finger markings correspond in detail as far as but not including the 

 minutise. 



4. Symmetrical double monsters (diplopagi) are closely related to the 

 last, and result from a partial, instead of a total separation, of the first 

 two blastomeres, the separation being sufficient to cause a loss of con- 

 tinuity and hence of relation, over a greater or less extent of surface. 

 The components of such monstrous births are the physical duplicates of 

 one another, and will doubtless be found to correspond in regard to 

 palm, sole and finger configuration, as do separate duplicate twins. The 

 double monsters of which we have authentic record are sufficiently num- 

 erous and diverse to represent every stage from that, of an otherwise 

 normal individual . with a doubling of certain of the median parts to 

 that of two complete duplicate twins with a slight connection between 

 them. They may also be arranged to represent several developmental 



