PLATE 5: EXPLANATION OF P'IGURES (CONTINUED) 



wandered into the nerve anlage form a sheath around the fibriUar part of the 

 nerve. The section shows a Rohon-Beard cell with neuraxon enterinjo; the root 

 of the dorsal nerve — a rather exceptional relation. 



29 A cross-section of a spinal nerve (MM 21-2-4) of a 46 mm. Squalus embryo, 

 showing the fibrillar portion of the nerve consisting of deeply stained fibrils and 

 a surrounding layer of sheath cells, presumably of medullary origin. The neu- 

 raxon fibers of the sensory bundle are somewhat larger in diameter than those 

 of the motor bundle. Both appear as highly refractive granules, separated by 

 a somewhat vacuolated intercellular plasma. 



30 A cross-section of a spinal nerve in a 17 mm. embryo, showing sensory and 

 motor components at the level of a sympathetic ganglion. The motor bundle 

 lies nearer the myotome than does the sensory bundle, and each is surrounded 

 by a single layer of sheath cells. The sympathetic ganglion anlage lies median 

 to and somewhat between the two. 



174 



