I20 EARLY EMBRYOLOGY OF THE CHICK 



communicans, which extends ventrad to a ganglion of the sym- 

 pathetic nerve cord. 



When first formed from the neural crest cells, the spinal 

 gangHon has no connection with the cord (Fig. 37). The dorsal 

 root is established by the growth of nerve fibers from cells of 

 the spinal gangHon mesiad into the dorsal part of the lateral 

 plate of the cord. At the same time fibers grow distad from 

 these cells to form the peripheral part of the nerve (Fig. 44). 

 The fibers which arise from the dorsal root ganglion conduct 

 sensory impulses toward the cord. 



Coincident with the estabhshment of the dorsal root, the 

 ventral root is formed by fibers which grow out from cells 

 located in the ventral part of the lateral plate of the cord 

 (Fig. 44). The fibers which thus arise from cells in the cord 

 and pass out through the ventral root, conduct motor impulses 

 from the brain and cord to the muscles with which they are 

 associated peripherally. 



The sympathetic ganglia arise from cells of the neural crest 

 which migrate ventrally and form cellular masses lying on 

 either side of the mid-line at the level of the dorsal aorta. 

 By the end of the fourth day these cells constitute a pair of 

 cords in which enlargements can be made out opposite the spinal 

 ganglia. These enlargements are the primary sympathetic 

 ganglia. Each sympathetic ganglion is connected with the 

 corresponding spinal nerve by a cellular cord which is the 

 primordium of the ramus communicans. The sympathetic 

 ganglia later receive both sensory and motor fibers from the 

 spinal nerve roots by way of the rami communicantes, and from 

 nerve cells in the sympathetic ganglia, fibers extend to the 

 viscera. 



111. The Sense Organs 



The Eye. — The primary optic vesicles arise in chicks of about 

 30 hours as dilations in the lateral wall of the prosencephalon 

 (Figs. 19 and 23). At first the optic vesicles open broadly 

 into the brain, but later constrictions develop which narrow 

 their attachment to the form of a stalk (Fig. 22). In chicks 

 of 55 hours the primary optic vesicles are in vagina ted to form 

 the double-walled secondary optic vesicles or optic cups. The 

 invagination takes place in such a way that the ventral wall 



