ORGANIC SYMMETRY IN ARMADILLO QUADRUPLETS. 



177 



bony condition will be like that in 36. Similarly when the notch 

 is shallow as in 50, we might expect the bony condition to be as 

 in 5&. 



It is an open question as to which doubling is more fundamen- 

 tal, that of the dermal plate or that of the horny scute. Onto- 

 getically the scute is the older structure, but I am inclined to 

 look upon the two structures as phylogenetically coeval, for we 

 apparently have in the armadillo a case of the persistence in a 



16 20 36 45 5 



Q/l f\QOf\ f\ QOQ 00 00 



6b 



FIGS. 1-8. 



mammal of the ancestral reptilian epidermal scale with its dermal 

 bony core. The hair group that centers about the scute is 

 presumably of more recent origin. 



I look upon scute and plate doubling as cases of local budding 

 of normally single primordia, not unlike the division of scutes 

 and plates in the carapace of modern tortoises, a phenomenon 

 that has received considerable attention (Newman, '05, and 

 Coker, '10). The physiological basis of this budding may be a 

 localized lowering of the rate of growth during scute develop- 

 ment. If we had a solution of the problem of budding or division 



