﻿84 
  INTRODUCTION. 
  

  

  figure 
  from 
  Ecker's 
  ' 
  Icones 
  Phjsiologicae 
  ' 
  will 
  give 
  

   an 
  idea. 
  

  

  This 
  type 
  of 
  segmentation, 
  in 
  which 
  the 
  first 
  furrow 
  

   completely 
  cleaves 
  the 
  whole 
  egg 
  into 
  two, 
  each 
  suc- 
  

   cessive 
  one 
  again 
  subdividing 
  that 
  cell 
  with 
  which 
  

   it 
  is 
  related, 
  the 
  first 
  two 
  furrows 
  vertical 
  and 
  the 
  

   third 
  horizontal, 
  is 
  termed 
  complete 
  or 
  holohlastic. 
  

   The 
  first 
  plane 
  of 
  division 
  passes 
  through 
  the 
  plane 
  of 
  

   symmetry 
  of 
  the 
  future 
  embryo, 
  the 
  tissues 
  of 
  which 
  

   are 
  formed 
  from 
  these 
  cells. 
  The 
  egg 
  of 
  Alytes 
  

   contains 
  a 
  much 
  greater 
  quantity 
  of 
  nutritive 
  matter, 
  

   and 
  the 
  segmentation 
  is 
  less 
  regular 
  and 
  more 
  limited 
  ; 
  

   it 
  belongs 
  to 
  the 
  merohlastic 
  type. 
  

  

  The 
  time 
  required 
  for 
  the 
  eggs 
  to 
  hatch 
  varies 
  con- 
  

   siderably, 
  according 
  to 
  the 
  species. 
  Thirty 
  hours 
  

   may 
  suffice 
  for 
  those 
  of 
  Discoglossiis 
  ])ictiis, 
  whilst 
  

   those 
  of 
  Ahjtes 
  ohstetricans 
  take 
  at 
  least, 
  under 
  the 
  

   best 
  conditions, 
  nearly 
  three 
  weeks. 
  The 
  eggs 
  of 
  

   other 
  species 
  fall 
  between 
  the 
  two 
  extremes, 
  the 
  

   evolution 
  being 
  of 
  course 
  accelerated 
  or 
  retarded 
  by 
  

   the 
  variable 
  temperature 
  with 
  which 
  early 
  breeders 
  

   have 
  to 
  contend, 
  or 
  even 
  temporarily 
  arrested 
  when 
  

   the 
  thermometer 
  descends 
  to 
  freezing-point 
  or 
  below. 
  

  

  