﻿74 
  

  

  Salensky's 
  hypothesis 
  seems 
  to 
  be 
  almost 
  identical 
  with 
  Ray 
  

   Lankester's 
  so-called 
  "planula 
  theory," 
  published 
  the 
  year 
  

   before 
  (On 
  the 
  Primitive 
  Cell-layers 
  of 
  the 
  Embryo 
  as 
  a 
  Basis 
  

   of 
  Genealogical 
  Classification 
  of 
  Animals. 
  Ann. 
  and 
  Mag. 
  

   Nat. 
  Hist., 
  May, 
  1873 
  : 
  and, 
  Notes 
  on 
  Embryology 
  and 
  Classi- 
  

   fication, 
  for 
  the 
  use 
  of 
  Students. 
  London 
  : 
  1877), 
  the 
  essential 
  

   difierence 
  between 
  wdiich 
  and 
  Haeckel's 
  gastrula 
  theory 
  is 
  

   the 
  conception 
  of 
  the 
  gastrula 
  as 
  a 
  secondary 
  modification 
  of 
  

   the 
  planula. 
  The 
  fact 
  that 
  in 
  Unio 
  and 
  in 
  the 
  oyster 
  a 
  plan- 
  

   ula 
  is 
  formed 
  by 
  the 
  modification 
  of 
  a 
  gastrula, 
  would 
  seem, 
  

   so 
  far 
  as 
  it 
  goes, 
  to 
  be 
  as 
  adverse 
  to 
  Lankester's 
  hypothesis 
  

   as 
  it 
  is 
  to 
  Salensky's. 
  A^on 
  Jhering's 
  view, 
  that 
  the 
  primitive 
  

   embryonic 
  form 
  among 
  the 
  Mollusca 
  is 
  not 
  a 
  gastrula, 
  but 
  a 
  

   " 
  leposphaera," 
  does 
  not 
  seem 
  to 
  involve 
  any 
  new 
  conception, 
  

   for 
  according 
  to 
  his 
  definition 
  (Ueber 
  die 
  Ontogenie 
  von 
  Cy, 
  

   clas, 
  und 
  die 
  Homologie 
  die 
  keimblatter 
  bei 
  den 
  Mollusken, 
  

   von 
  Dr. 
  Hermann 
  von 
  Jhering. 
  Zeitschr. 
  f. 
  Wiss. 
  Zool. 
  xxiv. 
  

   Marz. 
  1876, 
  pp. 
  414-438), 
  his 
  leposphaera 
  is 
  the 
  same 
  as 
  

   Lankester's 
  and 
  Salensky's 
  planula. 
  " 
  Die 
  Leposphaera 
  wird 
  

   also 
  aus 
  zwei 
  concentrischen 
  Zellschichten 
  gebildet 
  von 
  denen 
  

   die 
  aussere 
  oder 
  das 
  primare 
  Ectoderm 
  die 
  inn 
  ere 
  oder 
  das 
  pri- 
  

   mare 
  Endoderm 
  umgiebt, 
  wie 
  die 
  Schalle 
  einer 
  Nnss 
  den 
  Kern 
  

   einschliesst. 
  Der 
  -bleibende 
  Mund 
  entsteht 
  im 
  Ectoderm 
  der 
  

   Leposphaera, 
  der 
  Oesophagus 
  entweder 
  vom 
  Munde 
  aus, 
  wie 
  

   bei 
  den 
  Gasteropoden, 
  oder 
  vom 
  primare 
  Endoderm 
  aus, 
  wie 
  

   bei 
  den 
  Lamellibranchien," 
  p. 
  429. 
  

  

  The 
  facts 
  in 
  the 
  development 
  of 
  the 
  oyster 
  are 
  thus 
  seen 
  to 
  

   be 
  opposed 
  to 
  the 
  only 
  consistent 
  and 
  probable 
  hypothesis 
  

   which 
  has 
  ever 
  been 
  proposed 
  in 
  the 
  place 
  of 
  the 
  gastrula 
  

   theory 
  ; 
  the 
  hypothesis 
  that 
  the 
  planula 
  is 
  the 
  primitive 
  em- 
  

   bryonic 
  form, 
  of 
  which 
  the 
  gastrula 
  is 
  a 
  speciaHzed 
  modifica- 
  

   tion 
  ; 
  but 
  it 
  must 
  not 
  be 
  concluded 
  that 
  since 
  the 
  embryology 
  

   of 
  the 
  Mollusca 
  is 
  opposed 
  to 
  the 
  alternative 
  hypothesis 
  it 
  

   therefore 
  tends 
  to 
  support 
  the 
  hypothesis 
  that 
  the 
  gastrula 
  

   stage 
  has 
  a 
  phylogenetic 
  significance, 
  and 
  shows 
  the 
  descent 
  

   of 
  the 
  Mollusca 
  from 
  an 
  ancestral 
  gastraea. 
  

  

  The 
  most 
  satisfactory 
  and 
  trustworthy 
  papers 
  which 
  we 
  

   possess 
  upon 
  the 
  development 
  of 
  the 
  Mollusca 
  show 
  that 
  

  

  