32 THE PERIODIC GROWTH OF SCALES IN GADID^E 



Ryder's hypothesis seeks to account for the arrangement of scales in longi- 

 tudinal and oblique rows in two directions, and for their state of imbrication. 

 Scales are arranged in oblique rows showing two directions: (1) a direction 

 from above downward and backward ; (2) in the reverse direction, from below 

 ui)ward and backward. The scales may thus be enumerated in three different 

 directions: (1) in a downward and forward direction; (2) in a downward and 

 backward direction ; and (3) starting from any scale in any oblique row, they 

 may be counted either forward or backward longitudinally in the direction of 

 the long axis of the fish. In archaic types, the number of scales in a longi- 

 tudinal row on the sides of the body corresponds very exactly with the number 

 of muscle-plates or somites of the body. The myocommata, or sheets of 

 connective tissue intervening between the successive muscle-plates are attached 

 with great firmness to the deeper layers of the skin or corium. The structural 

 arrangements at the time of scale development noted above, affect and modify 

 the subsequent growth of the scale matrix. During the swimming movements 

 of the fish the entire integument is thrown into definitely circumscribed 

 areolce, the central portions of which remain in a passive condition, while the 

 periphery is wrinkled and folded as a result of the action of the lateral muscles 

 of the fish. In this way each and every one of the dermal and epidermal 

 areolae are circumscribed by the action of the fish in the normal act of swim- 

 ming. In each of the circumscribed areolte a scale develops ; the continuity of 

 its development with its fellows across the margins of the areolae is prevented 

 by the continual bendings or flexures to which the dermis is there subjected 

 owing to the action of the muscles. As it is impossible to state clearly the 

 details of Ryder's paper without also giving his drawings, I will content my. 

 self with quoting several of his sentences. 



" It will be clear that in the case considered the arrangement and imbrica- 

 tion of the body is determined by the actions of the segmentally arranged 

 muscles of the body. In other words, Avhatever has determined the develop- 

 ment of somites has also, in the most clear and direct manner, determined 

 the segmentally recurrent and peculiar trilinear and imbricated arrangement 

 of the scales of many fishes. It may be urged that heredity has determined 

 the number, arrangement, and the development of the somites, and therefore 

 the development of the scale is also a sequence of hereditary influences 

 working thus indirectly. Tliis view of the .case may be admitted without 

 invalidating the conclusion that given the growing mechanism here described, 

 the development of the scale would, under any circumstances, have been 

 interfered with at the parts where the integument was being continually flexed, 

 wrinkled, or folded, as it is around the integumentary areolas wherein the 

 scales are formed, as has been here proved to correspond with the facts." 



Ryder summaries " two conclusions of prime importance : — 



"(1) The scales of fish bear a segmental relation to the remaining hard and 

 soft parts, and are either repeated consecutively and in oblique rows corre- 

 sponding to the number of segments, or they may be repeated in rows as 

 multiples of the somites, or segmental reduction may occur which may effect 

 the arrangement of the scales so as to reduce the number of rows below the 

 number of somites indicated by the other soft and hard parts. 



