158 CHARLES MIDLO AND HAROLD CUMMIXS 



velopment. Prior to and at the time of ridge differentiation 

 the previously more prominent volar pads are in process of 

 involution. A pad which remains prominently elevated and 

 more or less definitely circumscribed will determine, by virtue 

 of its local complex of growth stresses, the formation of a 

 definite pattern, its specific character varying in accord with 

 the contour and circumscription of the pad. 



Valleys in the volar reliefs, as instanced by the sunken 

 parathenar area, like the pads elevated from the surface, 

 represent foci of local irregularities in growth. Hence they 

 are potentially seats of production of patterns or of regional 

 irregularities in ridge direction. 



A triradius is formed at the point of junction of three local 

 growth complexes, its triangular composition indicating the 

 action of forces from three directions. The topography of 

 triradii on the palm and sole follows a fairly consistent plan. 

 This plan is discussed by Whipple from the standpoint es- 

 pecially of the arrangement of triradii in reference to pads 

 and to the skin folds which adjoin pads in many mammals. 

 Schlaginhaufen constructs an elaborate scheme of the place- 

 ment of triradii and he emphasizes their variations among 

 the primates; Bychowska follows the same scheme in her 

 analysis of the primate palm. From the view of developmental 

 mechanics the basis of a morphologic plan of the triradii is 

 obvious. The mutual conjunctions of eminences and their re- 

 lations with the bases of digits and other marginal irregulari- 

 ties agree in a general way throughout the primate scale, 

 inasmuch as there is a basic morphologic plan of the pads. 

 Since, however, modifications of pads bring about correlated 

 variations in their number and placement, the identities of 

 individual triradii are sometimes not readily determined. Our 

 decision to omit a consideration of these features in the pres- 

 ent study was based upon the belief that the fundamentally 

 important variations of dermatoglyphics are pattern expres- 

 sions, and that the triradii are secondary elements, often 

 present, absent, or displaced through fortuitous developmental 



