62 



NERVOUS SYSTEM OF VERTEBRATES. 



in that its cells of origin arise and remain in the ectoderm. These 

 are the sensory cells of the nasal epithelium. From the inner end 

 of each of these cells arises a fiber (Fig. 34) which grows toward 

 the forebrain and enters the bulbus olfactorius. These con- 

 stitute the olfactory nerve. The fibers are never myelinated. 

 Although so little is certainly known of the details of the part 

 played by ectodermal placodes in the formation of the sensory 

 nerves and ganglia, it is clear that the nervous ectoderm is not 

 limited to the neural plate. It is probable that the limits of the 

 ectoderm which give rise to the nervous system are indefinite and 

 variable. All that can be said is that the neural plate gives rise 

 to the spinal cord and brain with the neural crest, while the olfac- 

 tory organ and nerve and the ganglia and nerves of the acustico- 

 lateral system are derived from ectoderm adjoining the neural 

 plate. These ganglia and nerves, including the olfactory, are to 

 be compared with those derived from the neural crest. All are 



FIG. 35. Two figures representing the formation of unipolar cells in the spinal 

 ganglion of the dog embryo. After Van Gehuchten. r., root; p., peripheral nerve. 



derived from ectoderm which in the early embryo is closely adjacent 

 to the lateral border of that which forms the central nervous system, 

 and the neural crest is wanting in those segments in which the 

 olfactory and acustico-lateral systems develop. The acustico- 

 lateral ganglion cells remain in the general ectoderm for a time, 

 the olfactory permanently so. It is probable that the epibranchial 

 placodes, which are much farther removed from the neural plate, 

 give rise to a part of the visceral sensory nerves, but whether 

 general visceral or gustatory fibers are so formed is unknown. 

 In Amphioxus and cyclostomes the sensory ganglion cells 

 remain throughout life bipolar, spindle-shaped cells such as have 

 been described. During the course of development in true fishes 



