258 



THE DEVELOPMENT OF THE CHICK 



In the course of the fourth and fifth days aggregations of 

 sympathetic ganghon cells begin to appear ventral to the aorta, 

 and in the mesentery near the intestine. The connection of these 

 with the primary cord is usually rendered evident by agreement 

 in structure, and by the presence of intervening strands of cells; 

 moreover, in point of time they always succeed the primary cord, 

 so that their origin from it can hardly be doubted. 



About the sixth day the secondary or permanent sympathetic 

 trunk begins to appear as a series of groups of neuroblasts situ- 

 ated just median to the ventral roots of the spinal nerves. They 



l^efiAS. 



Fig. 152. — Reconstruction in the sagittal plane 

 of the anterior spinal and sympathetic gan- 

 glia of a chick embryo of 4 days. (After 

 Neumayer.) 

 Ceph. S., Cephalic continuation of the sym- 

 pathetic trunk. S. C, Sympathetic cord. Sg., 

 Sympathetic ganglion, sp., Spinal nerve, spg., 

 Spinal ganglion. R. C, Ramus communicans. 



are thus separated from the spinal ganglia only by the fibers 

 of the ventral roots between which neuroblasts may be found, 

 caught apparently in migration from the spinal to the sympa- 

 thetic ganglion. The number of these secondary sympathetic 

 ganglia is originally 30, one opposite the main vagus ganglion, 

 and each spinal ganglion to the twenty-ninth (Fig. 150). Soon 

 after their origin they acquire three connections by means of 

 axones, viz., (a) central, with the corresponding spinal nerve- 



