PALM AND SOLE STUDIES. 2IQ 



four hands and three of the feet correspond as closely, as has 

 been found in other cases of separate duplicates; but one of the 

 feet shows a series of departures from the type, all due to the 

 presence, or retention, of triradii lost in the other cases. Once 

 or twice in my other sets of apparently duplicate twins I have 

 found about the same degree of difference in one of the members, 

 which has caused me some doubts, and has weakened the argu- 

 ment in the judgment of others, notably Newman and Patterson, 

 1911, pp. 856-860. 



In this case, however, we have twins who are undoubtedly dupli- 

 cates, and in which the variation is marked in one member of the 

 eight involved, the others showing the typical correspondence in 

 comparing both the two individuals, and the two sides of the 

 same individual. The case is thus of great service in showing 

 the limit of hereditary control affecting a case where the in- 

 dividual differences are expressed as varying degrees of degener- 

 acy or loss of the features of a complex ancestral configuration. 



After the presentation of this new piece of evidence on the 

 subject of palm and sole correspondences in duplicate twins, it 

 would seem a fitting time to review briefly the question of the 

 origin of such cases, especially since a flood of light has been shed 

 upon the whole subject by the investigations of Newman and 

 Patterson on the armadillo. 



The early stages of normal human individuals during the time 

 in which we are to expect to find the initial stages of the doubling 

 process are practically unknown, and it would be an almost 

 inconceivable chance which would furnish the material for the 

 study of these stages in human eggs destined to produce twins. 

 We have long been wont, how r ever, to rely in such cases upon the 

 analogies furnished by related mammals, and to fill in the breaks 

 in our knowledge by investigation of material taken from other 

 forms. In this way the normal embryology of single human 

 embryos has been well made out, and in a way that is generally 

 accepted, although actual human material, covering important 

 periods of development, is thus far wanting. 



In the armadillo the egg is normally polyembryonic, always 

 producing monosexual individuals which, by the number and 

 arrangement of their scales, prove themselves to be duplicates. 



