THE MUSCLE-PLATES. 



sp.c 



layer, which in all cases is converted into muscles earlier than 

 the outer layer. 



The conversion of the muscle-plates into muscles. It 



will 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. 379, mp). The 

 outer or somatic wall is formed of 

 simple epithelial -like cells. The 

 inner or splanchnic wall has how- 

 ever a somewhat complicated struc- 

 ture. It is composed dorsally and 

 ventrally of a columnar epithelium, 

 but in its middle portion of the 

 muscle-cells previously spoken of. 

 Between these and the central cavity 

 of the plates the epithelium forming 

 the remainder of the layer com- 

 mences to insert itself; so that be- 

 tween the first-formed muscle and 

 the cavity of the muscle-plate there 

 appears a thin layer of cells, not 

 however continuous throughout. 



When first formed the muscle- 

 plates, as viewed from the exterior, 

 have nearly straight edges ; soon 

 however they become bent in the middle, so that the edges have 

 an obtusely angular form, the apex of the angle being directed 

 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 



FIG. 379. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM EM- 

 BRYO SLIGHTLY YOUNGER THAN 



28 F. 



sp.c. spinal canal ; W. white 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 x. sub-notochordal rod ; ao. aorta ; 

 mp. muscle-plate; mp' . inner layer 

 of muscle-plate already converted 

 into muscles ; Vr. rudiment of 

 vertebral body ; si. segmental 

 tube ; sd. segmental duct ; sp.v. 

 spiral valve ; z/. subintestinal vein ; 

 P.O. primitive generative cells. 



