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SCIENCE 



[N. S. Vol. XL^^:I. No. 1207 



eral as would be expected from the struc- 

 ture of their nets. Probably the only 

 evidence of polarity that these nets exhibit 

 is seen in the temporary condition that has 

 been claimed for them by von Uexkiill, 

 namely, that impulses flow for the moment 

 more freely through them into stretched re- 

 gions than into unstretched regions. Aside 

 from this momentary state they are prob- 

 ably quite unpolarized in their transmit- 

 ting capacities. 



From such a condition as this it ought 

 to be possible to trace the transition stages 

 that have led to the synaptic nervous sys- 

 tem, and, in fact, examples of this kind are 

 not difficult to find. As a first step in this 

 direction we may examine the tentacles of 

 sea-anemones. These organs were shown 

 by Eand (1909) to exhibit in their re- 

 sponses to stimulation a marked polarity. 

 If a stimulus is applied to the tip of a 

 tentacle, the whole tentacle usually short- 

 ens. If it is applied to any other point on 

 the tentacle, this organ shortens as a rule 

 only from the point stimulated to the base, 

 the distal portion of the tentacle remain- 

 ing unchanged. Hence it may be con- 

 cluded that transmission does not proceed 

 from any region in the tentacle freely in 

 all directions, but only towards its base ; in 

 other words, the tentacle exhibits polarity. 

 As this polarity disappears on treating the 

 tentacle with chloretone or other anesthe- 

 tizing agents, it is clear that it is a nervous 

 polarity. The neuromuscular mechanism 

 of the tentacle is well known to consist of 

 peripheral sense cells whose deep ends are 

 much branched constituting a nerve-net 

 that is applied to the longitudinal muscle 

 cells of the tentacular ectoderm. The 

 polarity of the tentacle depends upon a 

 peculiarity in the structure of the sense 

 cells as pointed out by Groselj (1909), 

 namely, that most of the fibrous prolonga- 

 tions from the deep ends of these cells, in- 



stead of spreading out in all directions, ex- 

 tend down the tentacle towards the base. 

 Hence, when the sense cells are stimulated, 

 nerve impulses are generated, which, in 

 consequence of the direction of the cell 

 fibers, are conducted into the proximal 

 portion of the tentacle, where they call 

 forth the contraction of the longitudinal 

 muscle cells. Here then is the first evi- 

 dence of permanent nervous polarity such 

 as is so clearly shown in the neurone. It 

 occurs in a nerve-net without synapses, but 

 so organized that its fibrous constituents, 

 instead of being diffusely arranged, have a 

 predominating trend in one direction. 



Judging from the nature of the re- 

 sponses, polarized nerve-nets occur in many 

 other places. Thus the stalk of the giant 

 hydrozoan, Corymorpha, has recently been 

 shown to transmit nervous impulses more 

 freely on its length than transversely, a 

 condition that immediately suggests a 

 locomotor waves that pass over the foot of 

 a creeping snail are believed with good rea- 

 son to depend upon the presence of a 

 nerve-net, in which case the net must be 

 strongly polarized, for these waves are 

 limited in almost every instance to a single 

 direction. In a similar way the peristalsis 

 of the vertebrate digestive tube implies a 

 polarized net in the wall of that structure. 



Thus the primitive, diffuse, or apolar, 

 nerve-net may be imagined to undergo the 

 first change toward a synaptic system by 

 becoming polarized, a process that may be 

 described as due to a lengthening of its 

 fibers in one direction, whereby transmis- 

 sion in that direction predominates over 

 transmission in any other. The cells whose 

 processes exhibit this change are the ordi- 

 nary sense cells and nerve cells of the 

 nerve-net. They may be looked upon as 

 the forerunners of neurones, protoneurones 

 so to speak, and from them have arisen by 

 further differentiation the highly special- 



