1HE ORGANS OF THE OUTER GERM-LAYER. 



Fijr. 2G2 Cross section through the anterior part of tie tr^nk of an embry. of ScyUium, 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, 

 (dv), from which the fund'onent of the pancreas (pan) is given off dorsally and tha funda- 

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

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



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

 limbs ; nl, nervus lateralis ; ao, aorta ; eh, chorda ;.sy.g, sympathetic ganglion ; ea.v, cardinal 

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



