THE ORGANS OF THE OUTER GERM-LAYER. 



461 



Fig. 262. Cross section through the anterior part of tie trunk of an embryo of Scyllium, after 

 BALFOUR. 



Between the dorsal wall of the trunk and its ventral wall, where the attachment of the stalk 

 of the yolk-sac is cut, there is stretched a thick richly cellular mesentery, which completely 

 divides the body-cavity into right and left halves. Within the mesentery the duodenum 

 (du), from which the fundament of the pancreas (pan) is given off dorsally and the funda- 

 ment of the liver (hp.d) ventrally, is twice cut through. In addition, the place where the 

 vitelline duct [umbilical canal] (umc) joins the duodenum is visible. 



sp.c, Spinal cord ; s.pg, ganglion of posterior root ; ar, anterior root ; dn, dorsally directed nerve 

 springing from the posterior root ; mp, muscle-plate ; -nip', part of the muscle-plate already 

 converted into muscles ; mp.l, part of the muscle-plate which gives rise to the muscles of the 

 limbs ; nl, nervus lateralis ; ao, aorta ; c/t, chorda ; sy.g, sympathetic ganglion; ca.v, cardinal 

 vein ; sp.n, spinal nerve ; sd, segmeutal (archiuephric) duct ; st, ssgmental tube. 



