556 



TILE MUSCLE-PL A TEX. 



verted into muscles at a very early period, and the outer layer for a 

 long time remains formed of indifferent cells. That these muscle- 

 plates give rise to the main muscular system of the trunk, at any 



in, 



eft. 



TI :n <; 



FIG. 380. TRANSVERSE SECTION THROUGH THE ANTEKIOK PART OF THE TRUNK 

 OF AN EMBRYO OF SCYLLIUM SLIGHTLY OLDKR THAN FIG. 2'J B. 



The section is diagrammatic in so fur that the anterior nerve-roots have been 

 inserted for the whole length; whereas they join the spinal cord half-way between two 

 posterior roots. 



sp.c. spinal cord; up. ft. ganglion of posterior root ; ar. anterior root; dn. dorsally 

 f.irected nerve springing from posterior root; mp. muscle-plate; mp'. part of ruuscle- 

 plate already converted into muscles; tn.pl. part of muscle-plate which gives rise to 

 the muscles of the limbs; 7. uervus lateralis ; ao. aorta; ch. notochord; sy.ij. sym- 

 pathetic ganglion; en. c. cardinal vein; sp.ii. spinal nerve; sd. segmeiital (archinephric) 

 duct; .s^. segmental tube; du. duodenum; pan. pancreas; Itp.d. point of junction of 

 hepatic duct with duodenum ; nine, umbilical canal. 



