SECTION OF COVER KD-EYED MEDUSAE. 83 



short duration, the blocking will most likely be 

 transitory. Even the slight btrains caused by hand- 

 ling a contractile strip in the air are generally 

 followed by a decrease in the rate of tlie waves, and 

 sometimes by their being completely blocked. Other 

 methods by which the passage of weaves in contrac- 

 tile strips admits of being blocked will be alluded 

 to farther on. 



Now, in all these cases of temporary blocking we 

 must conclude that when the contraction-waves 

 succeed in at last forcing a passage, some structural 

 change has taken place in the tissue at the region 

 of injury, corresponding with the functional change 

 of the re-establishment of physiological continuity. 

 The waves previously stopped at a certain point of 

 section or otherwise, after beating for a time on the 

 physiological barrier, are at last able to throw doAvn 

 the barrier, and thenceforward to proceed on their 

 way unhindered. What, then, is the nature of the 

 structural change which has taken place ? 



In the early days of this research, before the 

 presence of a nerve-plexus had been proved histo- 

 logically, I argued in favour of such a plexus on the 

 grounds furnished by many of the foregoing ex- 

 periments; and at a lecture given in the Royal 

 Institution I ventured to say that if a careful 

 investigation of the histology of these tissues should 

 fail to show the plexus which the result of those 

 experiments required me to assume, we should still 

 be compelled to suppose that the plexus w^as pre- 

 sent, although not sufficiently differentiated lo 

 admit of being seen. I further ventured to suggest 



