THE NERVOUS SYSTEM 127 



ably been derived from one of the olfactory type by a proximal migra- 

 tion of its cell body. The stimulation of its free-nerve terminals may 

 be conceived to take place, as Botezat has recently pointed out, through 

 the secretory activity of the surrounding epithelial cells as a result of 

 their contact with the stimulating solution, rather than from the direct 

 action of this solution on the nerve endings themselves. From this 

 standpoint the epithelium comes to be an essential element in the stim- 

 ulation of the neurone and affords, so to speak, a favorable sensory en- 

 vironment for the real nerve-endings. Finally, the gustatory neurones 

 may be said to have appropriated certain of these epithelial cells which 

 have become differentiated into taste buds and whose activity, probably 

 secretory in character, to follow Botezat, is called forth by the superim- 

 posed solution and is essential to the stimulation of the nerve endings. 

 Thus in the evolution of the chemical sense organs of vertebrates 

 certain integumentary cells originally quite independent of the recep- 

 tors came to be involved with these and were eventually appropriated by 

 them as essential parts of the gustatory apparatus. This process of ap- 

 propriation is not unlike that seen among the effectors and represents 

 one of the important steps by which the nervous system in the course of 

 its evolution has added to its complexity. Although the nervous system 

 probably arose in a scattered way at spots where the primitive multicel- 

 lular animal had developed muscle, it became unified through the need 

 for general transmission tracts, and, by increasing its own elements as 

 well as by appropriating additional effectors and receptors, it has im- 

 pressed upon the higher animals, including ourselves, a unity so pro- 

 found that it includes everything that we mean by personality. 



