Embryology of certain of the Lower Fishe>>. 209 



plasmic condition ; if activity is again set up, the fibril will 

 soon again form in response. The impulse can, of course, 

 pass through the simple, undifferentiated protoplasm, but it 

 can only do so with difficulty and in a relatively imperfect 

 fashion. For the transmission of intense impulses, probably 

 the completely developed nerve track or primitive nerve 

 fibril is an absolute necessity. 



On this view, what takes place when a nerve is cut 

 and regenerates is as follows: — The passage ceasing in the 

 peripheral part, the neurofibrillte revert to their simple 

 protoplasmic condition. Feeble impulses will still pass 

 however, even in the simple protoplasm. There may be 

 simple stray impulses affecting the peripheral portion, in 

 which case this may show a certain amount of regeneration. 

 Or if the end organ remains active with more or less of the 

 nerve undegenerated, faint attempts at the normal stimuli 

 will be able to pass across the degenerated bridge of 

 protoplasm between such peripheral part of the nerve and 

 the central stump. This will cause a re-formation of 

 primitive fibrils, which, being a mere expression of impulse 

 tracks, must necessarily form in continuity with the central 

 and peripheral parts of the track, in other words, the newly- 

 formed part of the fibril will be in absolute continuity with 

 the undegenerated part. 



Such a view of the origin of nerve fibrils is not without 

 interest from the psychological point of view, for we see how 

 if it be true new association paths may be readily laid down 

 in the brain, impulses passing at first with difficulty through 

 the undifferentiated cell substance, but their repetition 

 causing the tracks to be marked out as definite nerve fibrils 

 by which the transmission of impulse would be far more 

 perfectly carried out, but these paths tending in time, if not 

 used, to lapse back into the original protoplasmic condition. 



Genetic Affinities of Lower Gnathostomata. 



In working through sections of embryos of the various 

 groups of vertebrates, one is impressed by the remarkable 

 resemblances which appear on every hand between the 

 Dipnoi and the Amphibia urodela. This resemblance is one 



