Mechanical Genesis of the Scales of Fishes. 245 



whole depth of the body, then will somites T, IT, III, IV, V, 



&c. in the figure form two parallel series of muscular blocks 

 above and below tlie line a, b. Each half-somite is also seen 

 to present an acute apex directed backward at the points c and 

 </ above and below the line a, b. The somites I, II, III, IV, 

 and V are therefore sigmoid in outline as seen from the 

 exterior. The myo- 

 commataor connec- 

 tive-tissue septa 1, 

 2, 3, 4, 5, 6, &c., 

 which intervene be- 

 tween the somites, 

 have a correspond- 

 ing sigmoid arrange- 

 ment. The sigmoid 

 or ^-shaped myo- 

 commata and the 

 myotomes are recip- 

 rocally coadapted to 

 each other in con- 

 figuration, like a 

 nest of ^ :^ ^ ^'s 

 turned upon their 

 sides. If we fur- 

 ther supposed that 



thin- and thick-legged :^'s alternated thus — ^::^^^^:^ — 

 we might suppose the thin-legged ones to represent the myo- 

 commata and the thick-legged ones the muscle-plates or myo- 

 tomes. The muscular fibres of the thick-legged ^'s run 

 longitudinally from the posterior surface of the myocomraa 

 immediately in front of it, to be inserted into the anterior face 

 of the myocomma immediately behind it. The muscular 

 tension is therefore exerted npon the opposite sides of the 

 myocommata, and is thus propagated along the sides of the 

 body from the head to the tail, from the first to the last myo- 

 tome. But the tension upon the inner face of the skin is 

 along the lines of insertion of the myocommata 1 c'c^ 2 </c, 

 8 cfc, &e. ; this will serve to wrinkle the skin not only along 

 the lines 1 c'c, 2 c'c, &e., but also to cause wrinkles to appear 

 along the dotted lines d, d\ e, and e'. In tiiis way it is easy 

 to see that the whole lateral integument will be thrown into 

 definitely circumscribed rhomboidal areolae, in which separate 

 overlapping scales, s, s, s, .<?, may appear. The wrinkles thus 

 produced by the tension of myocommata upon the integuments 

 of the body will cross each other and be reinforced at six 

 points in the zigzag transverse course of each myocomma, viz. 

 Ann.iScMag.N.nist. Ser. 6. IW. xi. 18 



