THE ORGANS OF THE OUTER GERM-LAYEli. 



461 



Fig. 262. Cross section through the anterior part of tie tr-ink 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 directad nerve 

 springing from the posterior root ; mp, muscle-plate ; mp', 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 ; ch, chorda ; sy.g, sympathetic ganglion; ca.v, cardinal 

 vein ; sp.n, spinal nerve ; sd, segmental (archinephric) duct ; st, segmental tube. 



