CONCLUSION 207 



ity that is indicative of this specialized state is well known 

 to occur, however, in certain nerve-nets. For instance, 

 it has been pointed out that in the tentacles of sea-anem- 

 ones transmission is much more freely accomplished 

 in a proximal direction than in a distal one. This was 

 shown to result from the direction taken by the nerve- 

 fibrils in the nerve-net of the tentacles. Probably a sim- 

 ilar aggregation and alignment of fibers of the net into 

 definite tracts is responsible for the simple reflex by 

 which a stimulation of the tentacles of a sea-anemone by 

 food will cause its transverse mesenteric muscles to con- 

 tract and thus open its oesophagus. In some such way as 

 this a diffuse nerve-net may be pictured as converted 

 into the beginnings of a central nervous organ in that its 

 network of transmitting fibers can be supposed to be re- 

 arranged into systems of tracts whose connections are 

 such that transmission becomes at once limited and 

 specialized. 



But such a polarized derivative of a nerve-net is prob- 

 ably still far from even a simple central nervous organ. 

 Such a polarized nerve-net is to be met with in the my en- 

 teric plexus of the vertebrate intestine, but the remote- 

 ness of this structure from a central organ is well under- 

 stood when we compare its activities with those of the 

 vertebrate brain. The polarized nerve-net transmits in 

 either direction. Its chain of protoneurones carry im- 

 pulses up or down the series with indifference. This is 

 in strong contrast with the state of these cells when they 

 have arrived at the condition of neurones. It has long 

 been known that in the spinal cord of vertebrates im- 

 pulses may be sent over dorsal neurones into the cord to 

 reappear in ventral neurones and excite muscle contrac- 

 tion, but that it is impossible by stimulating ventral neu- 



