426 Duplicate Twins and Double Monsters 



auf den schon im Entstehen hegriffeneii Embryo." The italics are my 

 own and show that, in spite of what seems to be his inclination, Sobotta 

 can hardly be included in this section but belongs rather more to the 

 school of Valentine and Gerlach, who advance the period at which the 

 splitting takes place to some point considerably beyond that of the 

 early cleavage stages (II). 



The authors cited above are a small number of those who have ex- 

 pressed their views upon the subject, but it is hoped that the exposition of 

 theories includes practically all that are held upon the subject at the 

 present time. For farther information the reader may consult the 

 bibliography at the end of the paper, especially the papers of Wiedemann 

 and Windle, who enter into the history of the subject in some detail, 

 and also the general works on the subject. 



PAET II. 



STUDIES OP THE CONFIGURATION OP THE PRICTION-SKIN ^ (PALMS 

 AND SOLES) IN TWINS AND TRIPLETS. 



Scope of the Ixvestigation. 



The physical " identity " of duplicate twins, in so far as it has been 

 considered at all, has hitherto rested upon such data as facial expression, 

 color of hair and eyes, and physical proportions ; features which, although 

 perfectly reliable as far as they go, lack the definiteness necessary in a 

 scientific comparison. 



Facial expression is often strikingly similar in two brothers or two 

 sisters of separate birth, and, on the other hand, genuine duplicate twins 

 may be subjected during life to circumstances that differentiate them 

 to a marked degree. The same may be said of bodily measurement, 

 even those included in the Bertillon system, since, as will be shown, the 



^ The term " friction-skin " was proposed by me and used in a paper now 

 in press, written by my pupil, Miss Whipple. It designates that modified 

 form of skin found upon the contact surfaces of the ventral aspect of mam- 

 malian chiridia (hands and feet), which consists of ridges placed at right 

 angles to the direction of the most usual forces and designed to prevent slip- 

 ping after the manner of the milling or grooving in the handles of certain 

 instruments. The ridges themselves, which in the higher Primates cover 

 the entire ventral surface of the hands and feet, are called in the paper 

 referred to " friction-ridges," a term properly expressing their true function, 

 and hence not misleading as is the term " papillary ridges " hitherto used. 

 A full description of these terms and of the physiological action of the parts 

 involved will be found in Miss Whipple's paper. 



