210 THE ELEMENTARY NERVOUS SYSTEM 



union is, is not known, but that it is not a union of the 

 kind seen in the nerve-net is quite clear. As was indi- 

 cated in the discussion of synaptic transmission, the 

 union is not complete, for it transmits in only one of the 

 two possible directions. In picturing the conversion of 

 a nerve-net into a central nervous organ, the longer and 

 longer stretches over which nervous connections must be 

 established as the higher and larger animals have been 

 evolved must be kept in mind as a process which, as met 

 with in embryonic life, implies a growing together of dis- 



A 



FIG. 51. Stages in the differentiation of sense cells; A, sensory protoneurone from a 

 coelenterate; B, sensory neurone from a mollusk; C, primary sensory neurone from a verte- 

 brate. In each instance the peripheral end of the cell is toward the left, the central toward 

 the right. 



tantly located cells, a process that thus gives ample op- 

 portunity for the establishment of such relations as are 

 seen in the synapse and which, therefore, has been of fun- 

 damental importance in the evolution of the highest form 

 of nervous system. 



The types of cells that the synaptic nervous system 

 has received from the nerve-net are easily designated. 

 The most primitive nerve cell from the standpoint of ani- 

 mal phylogeny is the sense-cell, or receptive cell, such as 

 occurs in the sensory epithelium of the coelenterates (Fig. 

 51, A ). In this type of cell, the cell body with its nucleus 

 lies in the epithelial portion of the skin of such animals. 



