1892.] NATURAL SCIENCES OF PHILADELPHIA. 221 



the longitudinal, horizontal septum a, h. If we suppose the somites 

 of the adult as developed from a continuous embryonic segment 

 JT m ^ y W w: extending the whole 



depth of the body, then 

 will somites I, II, III, 

 IV, V etc., in the figure 

 form two parallel series 

 of muscular blocks 

 above and below the 

 line a, h. Each half 

 somite is also seen to 

 present an acute apex 

 directed backward at 

 the points c and c' 

 above and below the 

 line a, h. The somites 



1, II, III, IV and V are therefore sigmoid in outline as seen 

 from the exterior. The myocommata or connective tissue septa 1, 



2, 8, 4, 5, 6 etc., which intervene between the somites, have a corre- 

 sponding sigmoid arrangement. The sigmoid or :^-shaped myo- 

 commata and the myotomes, are reciprocally coadapted to each 

 other in configuration like a nest of :^ :^ ^ :^'s turned upon their 

 sides. If w^e further supposed that thin and thick-legged :^'s 

 alternated thus ^:^^^^^ we might suppose the thin-legged 

 ones to represent the myocommata and the thick-legged ones the 

 muscle plates or myotomes. The muscular fibres of the thick-legged 

 ^'s run longitudinally from the posterior surface of the myocomma 

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

 the myocomma immediately behiud it. The muscular tension is 

 therefore exerted upon 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 myotome. But the tension upon the 

 inner face of the skin is alonsr the lines of insertion of the mvocom- 

 mata 1 dc, 2 dc, 3 de, etc., this will serve to wrinkle the skin not 

 only along the lines 1 dc 2 dc, etc., but also to cause wrinkles to 

 appear along the dotted lines, cZ, d! , e and d. In this way it is easy 

 to see that the whole lateral integument will be thrown into definitely 

 circumscribed rhomboidal areola) in which separate overlapping 

 scales s, s, s, s, may appear. The wrinTvles thus produced by the 



