514 EMBRYOLOGY OF THE CHICK 



from the cord from this same ganglionic region which are known as 

 peripheral nerves. 



The ventral roots (Fig. 290) are formed by fibers growing out from 

 the lateral portions of the cord itself, and are thus efferent nerves carry- 

 ing motor impulses from the brain and spinal cord to the muscles. 



The sympathetic ganglia (Figs. 268, 290, 298) arise from cells which 

 have migrated ventrally from the neural crests to form cell masses on 

 each side of the midline on a level with the dorsal aorta. They are con- 

 nected to form cords, and on the fourth day enlargements can be seen 

 on these cords opposite the dorsal ganglia. These enlargements are 

 the primary sympathetic ganglia, each one of which is connected by a 

 ramus communicans to the corresponding spinal nerve. Later, both 

 sensory and motor fibers will extend to the sympathetic ganglia from 

 the spinal nerve roots as rami communicantes, while fibers running out 

 from the sympathetic ganglia connect with the various organs of the 

 body. 



THE ORGANS OF SPECIAL SENSE 



THE EYE ( Fig. 289) 



We have already discussed the projections from the fore-brain 

 which are to form the optic cups as well as how the ectoderm directly 

 opposite the optic cup thickens to form the lens, this lens then meeting 

 with the cup. 



On the fourth day the beginning of almost all the adult structures 

 of the eye can be seen. 



The thickened internal layer of the optic cup will give rise to the 

 sensory layer of the retina. 



The fibers which arise from the nerve cells in the retina grow along 

 the groove in the ventral surface of the optic stalk toward the brain to 

 form the optic nerve. 



The external layer of the optic cup will, become the pigment layer 

 of the retina. 



About the inside of the optic cup a grouping of mesenchymal cells 

 can be seen which gives rise to the sclera and the choroid coat. 



Some of the mesenchymal cells even make their way into the optic 

 cup through the choroid fissure, and give rise to the cellular elements 

 of the vitreous body. 



From the margins of the optic cup closest to the lens, the ciliary 

 apparatus of the eye is derived. 



From the superficial ectoderm which overlies the eye, the corneal 

 and conjunctival epithelium are derived. 



The mesenchymal cells which migrate to the region between the 

 lens and the corneal epithelium give rise to the substantia propria of 

 the cornea. The lens forms as a thickening of the superficial ectoderm, 

 which then becomes depressed so that it forms an invagination into the 



