142 OUTLINES OF CHORDATE DEVELOPMENT 



fibers grow out from the ganglion as the rudiment of the lateral 

 line nerves (Fig. 53), this branch is present throughout the tad- 

 pole stage but disappears at metamorphosis. The vagus gang- 

 lion also sends out processes which grow posteriorly, to the 

 thoracic and abdominal viscera; these form the visceral branch 

 of the X nerve. 



The branchiomeric cranial nerves include, beside the afferent 

 or sensory components, whose development has just been out- 

 lined, efferent or motor fibers in varying numbers. These do 

 not arise by a morphologically separate root, but neuroblasts 

 in the wall of the medulla send out processes (axons), which 

 leave the cord in association with the afferent roots described 

 above. They are distributed with the branches which pass 

 posterior to the gill clefts. 



The development of the III, IV, and VI cranial nerves is 

 incompletely known. They form, comparatively late, as out- 

 growths of neuroblasts located in the ventral parts of the med- 

 ulla. These processes extend through the mesenchyme of the 

 head into the orbit and innervate the muscles attached to the 

 eye-ball. The III nerve is the first to appear (Held) at a time 

 when 8-9 somites are present (5-6 mm.). 



B. THE SPINAL NERVES 



The spinal nerves differ from the cranial nerves in two 

 important respects: (a) they are related primarily with the 

 mesodermal somites instead of the visceral clefts, and (6) there 

 are no placodal elements connected with them. 



The neural crests continue from the head region as much 

 smaller strands of cells; these become broken into segments 

 which are the rudiments of the spinal ganglia. From the neu- 

 roblasts of each ganglion, cell processes grow out, some centrip- 

 etally into the cord, forming the dorsal root of the spinal nerve, 

 others centrif ugally, forming the peripheral afferent fibers which 

 are distributed chiefly to the skin and other sensory surfaces 

 (Fig. 46) . The ventral root of the spinal nerve is formed by out- 

 growths (axons) from neuroblasts in the ventral side of the cord. 



