﻿86 
  

  

  Fig. 
  2 
  represents 
  an 
  egg 
  of 
  the 
  clam 
  

   after 
  the 
  lapse 
  of 
  perhnps 
  two 
  hours 
  from 
  

   the 
  time 
  it 
  was 
  first 
  brought 
  into 
  contact 
  

   with 
  the 
  male 
  cell 
  ; 
  it 
  has 
  apparently 
  been 
  

   impregnated, 
  for 
  the 
  nnclens 
  and 
  nucleo- 
  

   lus 
  are 
  no 
  longer 
  visible. 
  It 
  has 
  also 
  be- 
  Figure 
  2. 
  

   come 
  perfectly 
  spherical 
  and 
  has 
  lost 
  the 
  membranous, 
  

   filmy 
  apj)endage 
  of 
  P^'ig. 
  1. 
  

  

  Fig. 
  3 
  exhibits 
  the 
  same 
  egg 
  still 
  later 
  

   {-y'Cv'll^ 
  when 
  the 
  first 
  polar 
  cell 
  has 
  appeared 
  as 
  a 
  

  

  SfH^Vf 
  «/. 
  -V// 
  ,;ii 
  minute 
  blister-like 
  prominence 
  at 
  one 
  side. 
  

  

  ^^S*"^"N^^-I'F' 
  '^^^^ 
  polar 
  cell 
  is 
  almost 
  transparent 
  and 
  is 
  

   ^^^^^•^ 
  situated 
  in 
  a 
  slight 
  depression, 
  from 
  which 
  

   Figure 
  3. 
  imx:)ressed 
  lines 
  radiate, 
  making 
  it 
  appear 
  

   as 
  if 
  the 
  vitelline 
  membrane 
  were 
  wrinkled 
  in 
  a 
  radial 
  

   manner. 
  x\ 
  similar 
  radial 
  wrinkling 
  around 
  the 
  polar 
  

   area 
  is 
  described 
  by 
  Whitman 
  as 
  occurring 
  during 
  the 
  

   formation 
  of 
  the 
  polar 
  cells 
  of 
  the 
  egg 
  of 
  a 
  small 
  fresh- 
  

   water 
  lee(*h 
  {Clepshie), 
  but 
  it 
  is 
  more 
  strongly 
  marked 
  

   than 
  in 
  the 
  Qgg 
  of 
  the 
  clam. 
  I 
  have 
  not 
  noted 
  this 
  radial 
  

   wrinkling 
  in 
  the 
  egg 
  of 
  the 
  oyster. 
  After 
  a 
  while 
  the 
  

   egg 
  represented 
  in 
  Fig. 
  3 
  undergoes 
  farther 
  (diange, 
  a 
  

   series 
  of 
  annular 
  waves 
  pass 
  from 
  the 
  pole 
  of 
  the 
  egg 
  

   opposite 
  the 
  polar 
  cell 
  and 
  a 
  second 
  jDolar 
  cell 
  is 
  pushed 
  

   out, 
  when 
  the 
  radial 
  wrinkles 
  disappear 
  and 
  the 
  polar 
  

   cells 
  present 
  the 
  appearance 
  of 
  two 
  small 
  spheres 
  joined 
  

   to 
  each 
  other 
  and 
  attached 
  to 
  the 
  egg 
  by 
  a 
  stalk 
  as 
  in 
  

   Fig. 
  0. 
  I 
  should 
  have 
  stated 
  , 
  before 
  that 
  the 
  first 
  polar 
  

   cell 
  originates 
  in 
  the 
  same 
  way 
  as 
  the 
  second 
  — 
  by 
  the 
  

   passage 
  of 
  a 
  sei-ies 
  of 
  waves 
  from 
  one 
  pole 
  to 
  the 
  other. 
  

   These 
  are 
  not 
  all 
  of 
  the 
  forces 
  that 
  are 
  at 
  w(U-k 
  in 
  the 
  

   exti-usion 
  of 
  the 
  polar 
  cell 
  ; 
  the 
  nucleus 
  of 
  the 
  egg 
  has 
  

   been 
  undei'going 
  changes 
  of 
  form, 
  elongating 
  and 
  con- 
  

   tracting, 
  disappearing 
  to 
  assume 
  a 
  spindle 
  form, 
  then 
  

   contracts 
  into 
  a 
  rounded 
  form, 
  but 
  with 
  one 
  end 
  in 
  inti- 
  

   mate 
  relation 
  with 
  the 
  polar 
  cells. 
  These 
  changes 
  of 
  

  

  