220 PROCEEDINGS OF THE ACADEMY OF [1892. 



ward and backward, and, starting from any scale in any oblique 

 row, they may be counted either forward or backward longitudinally 

 or in conformity with the direction of the axis of the body of the 

 fish. Tills is cons^iicuously the case in Clupeoids and some 

 Cyprinoids. 



In such archaic types as these, approximating the primitive iso- 

 spondylous condition, it is also found that the number of scales in a 

 longitudinal row corresponds, on the sides of the body, very exactly 

 with the number of muscle-plates or somites of the body. It is also 

 found that the rayocommata or sheets of connective tissue interven- 

 ing between the successive somites are attached with great firmness 



CD O 



to the deeper layers of the skin or corium. Such a construction, 

 together with the peculiar arrangement of the muscle plates at the 

 time the scales begin to develop conditions the further growth of 

 the scale matrix. This is affected in such a manner that the whole 

 of the integument is thrown into definitely circumscribed areolae, 

 during the ordinary movements of the fish in swimming. The 

 central portions of each of these areolae are left in a quiescent con- 

 dition while their margins are wrinkled or folded as a result of the 

 current action of the lateral muscles of the body. In this wise each 

 and every one of the dermal and epidermal areolse are circum- 

 scribed by the action of the fish in the normal act of swimming. In 

 each of the circumscribed areolse a scale develops; the continuity 

 of its development with its fellows across the margins of the areola 

 is prevented by the continual bendings or flexures to which the 

 dei'mis is there subjected owing to the action of the muscles. 



This will be better understood by referring to the accompanying 

 diagram rei:)resenting the arrangement of the muscular somites of 

 a Cyprinoid (Carasshis) with their intervening myocommata as 

 seen from the side when the skin with its scales is removed. Before 

 proceeding further, however, it may be well to insist upon the fact 

 that the rows of scales are found to conform to the successive somites. 

 This is of itself significant. The careful interpretation of the facts 

 from observation, however, discloses a very remarkable effect due to 

 the peculiar arrangement of the muscle plates. 



As is well known the muscular masses of the sides of the body of 

 a fish are arranged in the form of two longitudinal trihedral columns 

 separated along the middle line of the side a to 6 into a dorsal and 

 venfral half The somites entering into the composition of these 

 ventral and dorsal masses were at first absolutely continuous across 



