( 1952) as follows: 'In fact the two transitory first "layers" do not have the same value throughout the Vertebrata; they have been described as ectophylle and entophylle, two neutral (and therefore advantageous) terms proposed by Celestino da Costa ( 1947).' For comparable reasons Hadži ( 1949) has proposed the term mesohyle for mesoderm.

The theory of recapitulation and the germ-layer theory are each of them fallacious: their combination in the theory of the gastraea is all the more unacceptable.


Conclusions

The position is, therefore, that the germ layers do not equate perfectly either with the presumptive regions of the egg or blastula from which the various adult structures will arise, or with these structures when formed. It may well be asked, therefore, what value remains in the germ-layer theory. Looking at it from the point of view of causal embryology, there can be no doubt that the germ-layer theory was misconceived in its attempt to provide an embryological criterion of homology and in its assumption that the fates of the germ layers were universally equal and limited. 'The facts forbid us to see in these elementary organs of the embryo that definite predetermination for the performance of certain ontogenetic functions', wrote Jenkinson ( 1906). 'The germinal layers', he continued, 'are not sets of cells of universally identical origin which necessarily and invariably give rise to certain fixed parts of the adult organization.'

What, then, are the germ layers? In Oppenheimer's view they are 'the embryo's method of sorting out its constituent parts', and the problem consists in the elucidation of the limitation of the potencies of the germ layers to their normal accomplishment. Interesting as this problem is, it does not, however, appear to be exclusive to the germ layers. Rather should the germ layers be considered as a problem of the anatomy of embryos, and less as a problem of the production of the anatomy of the adult. Even allowing for the possibility that in the lowest Metazoa (Turbellaria and coelenterates), as mentioned above (p. 165), the germ layers have not become definitively established, in all animals above these, germ layers exist and they correspond in widely different types of organisms, regardless of the exact topographical localizations of the materials of which

-168-

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Embryos and Ancestors
Table of contents

Table of contents

  • Title Page iii
  • Preface v
  • Contents vii
  • List of Illustrations xi
  • I- Stages of Development And Stages of Evolution 1
  • II- Ontogeny 14
  • III- The Speeds of the Processes Of Development 22
  • IV- Phylogeny 29
  • V- Heterochrony 34
  • VI- Caenogenesis 40
  • Conclusion 51
  • VII- Deviation 52
  • Conclusion 61
  • VIII- Neoteny 63
  • Conclusion 90
  • IX- Vestigial Structures Due To Reduction 92
  • X- Adult Variation 97
  • XI- Vestigial Structures Due To Retardation 99
  • XII- Hypermorphosis 100
  • XIII- Acceleration 104
  • Conclusion 110
  • XIV- Paedomorphosis And Gerontomorphosis 111
  • XV- Repetition 125
  • XVI- Embryology and Taxonomy 134
  • XVII- Embryology and Homology 146
  • XVIII- The Germ Layers 154
  • Conclusions 168
  • XIX- Conclusions 170
  • Bibliography 175
  • Index 191
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