. The elasmobranch fishes. Chondrichthyes. 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
. The elasmobranch fishes. Chondrichthyes. 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 en
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