On the Mechanical Genesis of the Scales of Fishes. 243 



XLII. — On the Mechanical Genesis of the Scales of 

 Fishes, l^y JoiiN A. Ryder*. 



Fourteen years ago the present writer suggested that the 

 slow metamorphosis of the forms of the crowns of the teeth of 

 Mammalia t in the course of a vast number of successive 

 generations might be ascribed to the continuous, slow, and 

 cumulative action of mechanical strains and pressures in 

 definite directions, resulting in the production of permanent 

 stresses and consequent changes in the forms of the crowns, 

 especially of the molar series. The evidence since accumu- 

 lated from vertebrate palaeontology and anatomy has served 

 to strengthen the belief that such an hypothesis cannot be 

 dismissed as useless until a better one has been offered in its 

 stead. The present paper is an attempt to apply somewhat 

 analogous reasoning to a somewhat simpler but no less inter- 

 esting problem in morphogenesis. 



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 basement-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 

 Batrachus 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-membrane 

 from which the actinotrichia or primordial fin-rays of embryo 

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

 almost coextensive 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 direc- 

 tion 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 down- 

 ward and forward as well as downward and backward, and, 



* From the ' Proceedings of the Academy of Natural Sciences of 

 Philadelpliia,' 1892, pp. 219-224. 



t " On the Mechanical Cit-uesis of Tooth-forms,"' Proc. Acad. Nat. Sci, 

 Philad. 1878. 



