554 



THE MUSCLE. PLATES. 



features in their development, which may be regarded as primitive, 

 and as having been inherited from the ancestors of Vertebrates. 

 These features are (1) the extension of the body cavity into the 

 vertebral plates, and subsequent enclosure of this cavity between the 

 two layers of the muscle-plates ; (2) the primitive division of the 

 vertebral plate into an outer (somatic) and an inner (splanchnic) 

 layer, and the formation of a large part of the voluntary muscular 

 system out of the inner layer, which in all cases is converted into 

 muscles earlier than the outer layer. 



The conversion of the muscle-plates into muscles. It \\ill be 

 convenient to commence this subject with a description of the changes 



which take place in such a simple type 

 as that of the Elasmobranchii. 



At the time when the muscle-plates 

 have become independent structures 

 they form flat two-layered oblong bodies 

 enclosing a slit-like central cavity (fig. 

 279, nip}. The outer or somatic wall 

 is formed of simple epithelial-like cells. 

 The inner or splanchnic wall has how- 

 ever a somewhat complicated structure. 

 It is composed dorsally and ventrally 

 of a columnar epithelium, but in its 

 middle portion of the muscle-cells pre- 

 viously spoken of. Between these and 

 the central cavity of the plates the epi- 

 thelium forming the remainder of the 

 layer commences to insert itself; so 

 that between the first-formed muscle 

 and the cavity of the muscle-plate there 

 SECTION THROUGH appears a thin layer of cells, not how- 

 ever continuous throughout. 



When first formed the muscle-plates, 

 s viewed from the exterior, have nearly 



FIG. 379. 

 THE TRUNK OF A ScYLLIUM EMBRYO 

 SLIGHLY YOUNGER THAN 28 F. 



, Pe 



rior nerve-roots ; e/i. notochord ; straight edges; soon however they be- 



come bent in the middle, so that the 



.r. sub-notochordal rod; no. aorta; 



tube; sd. segrnental duct; sp.v. 

 spiral valve ; v. subintestiual vein ; 

 p.o. primitive generative cells. 



lges have an obtusely angular form 



into muscles; T>. rudiment of the apex oi the angle being directed 

 vertebral body; st. segmental forwards. They are so arranged that 



the anterior edge of the one plate fits 

 into the posterior edge of the one in 

 front. In the lines of junction between 

 the plates layers of connective tissue cells appear, which form the 

 commencements of the intermuscular septa. 



The growth of the plates is very rapid, and their upper ends 

 soon extend to the summit of the neural canal, and their lower ones 

 nearly meet in the median ventral line. The original band of 

 muscles, whose growth at first is very slow, now increases with great 



