1892.] NATURAL SCIENCES OF PHILADELPHIA. 219 



ON THE MECHANICAL GENESIS OF THE SCALES OF FISHES. 

 BY JOHN A. RYDEPv. 



Fourteen years ago the present writer suggested that the slow 

 metamorphosis of the forms of the crowns of the teeth of mammalia/ 

 in the course of a vast number of successive generations, might l)e 

 ascribed to the continuous, slow and cumulative action of mechanical 

 strains and pressures in definite directions, resulting in the produc- 

 tion of permanent stresses and consequent changes in the forms of 

 the crowns, especially of the molar series. The evidence since 

 accumulated from vertebrate palaeontology and anatomy has served 

 to strengthen the belief tliat such an hypothesis cannot be dismissed 

 as useless until a better one has been offered in its stead. The pre- 

 sent paper is an attempt to apply somewhat analogous reasoning to 

 a somewhat simpler, but no less interesting, problem in morpho- 

 genesis. 



The mechanical hypothesis now to be offered respecting the genesis 

 of the scales of fishes, accounts for the origin of such scales from a 

 continuous subepidermal matrix, which may be regarded as a base- 

 ment membrane. Such a matrix is found to actually exist in some 

 forms, at an early stage, just beneath the epidermis. It is thickest 

 on the dorsal and lateral aspects of the body as is seen in sections 

 of the young of the scaleless Batrachns tau, for example. Such a 

 matrix also exists in the larval stages of other scale-bearing forms 

 and may be continuous with the very attenuated basement mem- 

 brane from which the actinotrichia or primordial fin-rays of embryo 

 fishes seem to be in part differentiated. Such a matrix is almost 

 co-extensive with the whole epidermal layer of the young of many 

 types of fishes, just at the time when the scales commence to be 

 developed. 



The hypothesis further accounts for the arrangement of the scales 

 in longitudinal and in oblique rows in two directions. The oblique 

 rows are arranged, as is well-known, in a direction from above 

 downward and backward and also in the reverse direction from 

 below upward and backward. That is, the scales may be counted 

 in rows in three directions downward and forward as well as down- 



1 On the Mechanical Genesis of Tooth -forms. Proc. Acad. Nat. Sciences, 1878. 



