﻿618 
  GKORGE 
  WAGNER. 
  

  

  tacles 
  discharged 
  their 
  nematocysts 
  when 
  touched 
  by 
  a 
  particle 
  

   of 
  some 
  digestible 
  substance, 
  but 
  not 
  when 
  sand 
  was 
  allowed 
  

   to 
  fall 
  on 
  them. 
  Although 
  he 
  found 
  the 
  same 
  thing 
  true 
  of 
  

   an 
  isolated 
  tentacle 
  he 
  expressed 
  the 
  opinion 
  that 
  nematocyst 
  

   discharge 
  depended 
  on 
  the 
  will 
  of 
  the 
  animal. 
  

  

  I 
  tried 
  similar 
  experiments 
  with 
  a 
  great 
  many 
  Hydras 
  of 
  

   various 
  species, 
  and 
  in 
  no 
  case 
  was 
  I 
  able 
  to 
  secure 
  a 
  dis- 
  

   charge 
  of 
  nematocysts 
  by 
  mechanical 
  stimulation 
  alone. 
  

   Large 
  specimens 
  of 
  Hydra 
  grisea, 
  in 
  which 
  the 
  nematocysts 
  

   could 
  easily 
  be 
  recognised 
  under 
  the 
  Brauns-Driiner 
  micro- 
  

   scope, 
  were 
  stimulated 
  by 
  means 
  of 
  a 
  capillary 
  glass 
  rod. 
  

   The 
  tip 
  of 
  the 
  rod 
  was 
  moved 
  over 
  the 
  body 
  in 
  all 
  directions, 
  

   often 
  with 
  pressure 
  enough 
  to 
  produce 
  a 
  wound. 
  But 
  even 
  

   after 
  five 
  or 
  ten 
  minutes 
  of 
  such 
  treatment 
  no 
  discharge 
  of 
  

   nematocysts 
  results. 
  There 
  is 
  such 
  a 
  discharge 
  when 
  Hydra 
  

   is 
  crushed 
  under 
  a 
  cover 
  glass. 
  This 
  discharge 
  is 
  due, 
  

   however, 
  to 
  the 
  direct 
  pressure 
  brought 
  to 
  bear 
  on 
  the 
  cnido- 
  

   blasts, 
  and 
  has 
  no 
  bearing 
  on 
  the 
  normal 
  reaction, 
  as 
  it 
  is 
  in 
  

   no 
  sense 
  a 
  vital 
  phenomenon. 
  It 
  is 
  clear, 
  therefore, 
  that 
  

   mechanical 
  stimuli 
  are 
  not 
  adequate 
  for 
  nematocyst 
  discharge. 
  

   The 
  cnidocils 
  must, 
  therefore, 
  serve 
  a 
  function 
  different 
  from 
  

   that 
  suggested 
  by 
  their 
  common 
  name 
  of 
  "trigger." 
  

  

  Chemical 
  stimuli 
  was 
  next 
  resorted 
  to. 
  Various 
  chemicals 
  

   were 
  used, 
  such 
  as 
  acetic 
  acid, 
  methylene 
  blue, 
  citric 
  acid, 
  and 
  

   methyl 
  green. 
  It 
  was 
  found 
  that 
  all 
  of 
  these, 
  if 
  in 
  solutions 
  

   of 
  proper 
  strength, 
  would 
  cause 
  nematocyst 
  discharge. 
  Even 
  

   beef 
  tea, 
  when 
  strong 
  enough, 
  had 
  the 
  same 
  effect. 
  The 
  

   most 
  certain 
  in 
  action, 
  however, 
  and, 
  on 
  account 
  of 
  its 
  colour, 
  

   the 
  easiest 
  to 
  observe, 
  was 
  methyl 
  green. 
  If 
  a 
  small 
  amount 
  

   of 
  solid 
  methyl 
  green 
  was 
  applied 
  to 
  a 
  Hydra 
  in 
  the 
  manner 
  

   previously 
  described 
  (pp. 
  603 
  to 
  606) 
  there 
  was 
  always 
  a 
  dis- 
  

   charge 
  of 
  nematocysts, 
  provided 
  the 
  solution 
  of 
  the 
  stain 
  was 
  

   not 
  too 
  dilute 
  when 
  it 
  reached 
  the 
  Hydra. 
  This 
  discharge 
  is 
  

   absolutely 
  limited 
  to 
  the 
  area 
  directly 
  touched 
  by 
  the 
  stain. 
  

   After 
  the 
  stain 
  had 
  acted 
  a 
  short 
  time 
  the" 
  surplus 
  was 
  

   removed 
  by 
  a 
  pipette 
  or 
  the 
  Hydra 
  was 
  placed 
  in 
  another 
  

   watch-glass. 
  It 
  was 
  then 
  easily 
  seen 
  that 
  the 
  ectoderm 
  cells 
  

  

  