PALM AND SOLE STUDIES. 223 



is, twins which have some postembryonal parts in common, and 

 are consequently joined together, the principle of budding, as 

 seen in the armadillo, furnishes some clues as to the possible 

 mechanism of the process. Even in normal twins certain of the 

 embryonal parts are held in common, as always in the armadillo, 

 but as these common parts are embryonic (trophoblast) and are 

 abandoned at birth, they are not apparent later. In double mon- 

 sters the common parts include more than trophoblastic material 

 and the components are consequently not liberated by the cutting 

 of the umbilical cord. 



In a morphological sense the bands of the armadillo carapace, 

 with their variations, and the friction ridges of human palms 

 and soles are partially or wholly homologous structures, so that 

 their use in determining the degree of similarity of twinned in- 

 dividuals is equally warrantable in both cases, while the results 

 may well be compared. Both deal with epidermic structures, 

 the probable homologs of reptilian scales, placed in rows; in 

 both are observed the similar phenomena of the forking and con- 

 sequent doubling of the lines thus formed. Even the double 

 scale of the armadillo may have its counterpart in a twin sweat- 

 pore, which indicates the composite nature of the unit to which 

 they belong. 



While, however, the study of the armadillo has the great and 

 undeniable advantage of allowing the study of the actual em- 

 bryonic conditions in each case, there are also certain marked 

 advantages in the study of the human palm and sole configuration 

 as a similar criterion of similarity of individuals, and in following 

 the action of heredity. In the first place there is (i) the far 

 greater complexity of pattern, giving an almost infinite number 

 of points for comparison. Here the fineness of the lines, and 

 their occasional participation in the formation of a complex 

 pattern, allows in number and arrangement, to say nothing of 

 the minutiae, that is, the composition of the individual ridges, a 

 complexity far surpassing anything met with in the carapace of 

 the armadillo, where the details concern only nine rows of scales, 

 with some 50-60 unit scales in a row. Another very important 

 advantage is seen in (2) the fact that the records of human twins 

 may easily be made complete, with prints of both parents, father 



