THE ORGANS OF THE INNER GERM LAYER. 



325 



-mp 



Tig. 185. Cross section through the anterior part of the trunk of an embryo of Scyllium, after 

 BALFOUR. 



Between the dorsal and ventral walls of the body, where the attachment of the stalk of the yolk- 

 sac is cut, there is stretched a broad mesentery which contains numerous cells and completely 

 divides the body-cavity into a right and a left half. The duodenum (du), lying iu the 

 mesentery, is twice cut through ; dorsally it gives rise to the fundament of the pancreas 

 (pan), ventrally to that of the liver (hp.d). Further, the place where the vitelline duct 

 (unic) emerges from the duodenum is to be seen, sp.c, Neural tube (spinal cord) ; sp.g, 

 ganglion of posterior root ; ar, anterior root ; dn, dorsally directed nerve springing from the 

 posterior root ; mp, muscle-plate ; rap 1 , part of muscle-plate already converted into muscles ; 

 mp.l, part of muscie-plate which gives rise to the muscles of the limbs ; nl, nervus lateralis ; 

 ao, aorta ; c/t, chorda ; sy.y, sympathetic ganglion ; ca.v, cardinal vein ; sp.n, spinal nerve ; 

 sd. segmeutal duct (duct of primitive kidney) ; st, segmental tube (pronephric tubule). 



