238 



THE ELASMOBRANCH FISHES 



of one to the other is greatlj^ altered. Burckhardt (1911) has estimated that 

 in Scymnus the grey to the white is as 1 to 17, wliile in man it is as 5 to 12. 



Peripheral Nervous System 



The i)eripheral nervous system in general, like that in Heptanclins, is made up 

 of cranial and spinal elements. Both the cranial and the spinal nerves origi- 

 nate from cells in or derived from the central nervous system. AVe may briefly 

 review their development in a type like Acanthias. 



At the time when the neural tube closes, the cells which make up its walls 

 form a single layer (fig. 218) . These cells later become pear-shaped and collect 



in groups in the region of the ventral horns. 

 In the adult the motor fibers extend from 

 these cells. 



The cpiestion has often arisen as to the 

 formation of the nerve fiber or axone. Is it 

 formed in place from preexisting proto- 

 plasmic strands, or is the axone an out- 

 growth from the cell body ? An examination 

 of a section by Neal (1914) taken from the 

 posterior region of the cord (fig. 218) shows 

 that the fiber or axone (ax.) here has just 

 reached the myotome (m.) and that its end, 

 which is the actively growing part of the 

 fiber, is rhizopod in appearance. As the 

 muscle bud grows outward toward the 

 fin, the fiber l)ecomes attached to it and is 

 drawn out with it, forming a so-called end 

 plate. Such a growing fiber is protected by 

 a thin covering, the neurilemma, and sometimes a medullary sheath is formed 

 between the neurilemma and the axis cylinder of the fiber. The axis cylinder 

 itself becomes differentiated into numerous fibrillae. 



In the sensory (dorsal) nerves an overgrowth, the neural crest, is pro- 

 duced on each side along the neural tube at the place of closure. From each 

 crest, dorsal root ganglia result {g., fig. 207) which migrate slightly farther 

 down the sides of the tubes. Composing these ganglia are multitudes of bipolar 

 cells, the fibers from which pass from both ends or poles of the cell. Several of 

 these cells (fig. 207, a to <:•) are here seen in different stages of development; 

 (c) represents the mature cell in which the two poles have fused at the base 

 into the stem of a T. One of the fibers thus formed by the arms of the T enters 

 the central system wliile the other extends outward and receives sensation. 



Fig. 218. Transverse section througli 

 developing cord, notochord (chd.), 

 and myotome (i5(. ), Squal2is acan- 

 thias. (From Neal.) 



ax., motor axone growing out to 

 myotome (m.) from cell in the neu- 

 ral tube. 



CEANIAL NEEVES 



The first cranial or olfactory nerve extends from the epithelium of the olfac- 

 tory capsule as a double nerve backward to the olfactory bulb. In extent it 



