GENERAL CHARACTERISTICS OF THE SENSORY SYSTEM 9 



Rath, 1888, 1895, 1896; Hanstrom, 1928). Although the sensory 

 system of vertebrates also contains some primary sense cells (e.g., the 

 rods, cones, and olfactory receptors), the taste receptors and auditory 

 receptors are specialized non-neural cells connected with neurons. 

 Possession of primary sense cells means that one cell does the work of 



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Fig. 2. Stages in the embryonic development of a hair. Tr, trichogen cell; 

 To, tormogen cell; N, bipolar neuron. (Redrawn from Krumins, 

 1952.) 



at least two; it performs the multiple function of detecting environ- 

 mental energy, transducing it, and initiating and transmitting im- 

 pulses to the appropriate ganglion. From the point of view of the 

 sensory physiologist, the primacy of the insect receptor is a boon 

 to investigations, since all events occur in the one cell. It is 

 generally believed, furthermore, that the primary neurons make their 



