V
HETEROCHRONY

WE have seen in previous chapters that the strengths of the internal factors of development can vary and exert their effects at different rates, with the result that the time of appearance of a structure can be altered. To this shifting along the time-scale the term heterochrony is applied. It is thus possible for two organs to reverse the order of their appearance in successive ontogenies, and, by varying the rates at which animals become mature, adult structures can be reduced to a vestige and discarded, or youthful structures can become adult and so be introduced into the phylogeny.

A word must be said about the term 'youthful'. From the time the egg is fertilized until the young animal emerges from the egg- membranes, i.e. hatches, it is known as an embryo, and this period is called embryonic. After hatching, and until it has assumed the adult form, the young animal is known as a larva, this period being called larval, and often characterized by remarkable adaptations to the environmental conditions in which the larva lives.

While originally the embryonic period was very short, as animals became more complicated in evolution they took longer to develop, which meant that the time at which the young animal was hatched and could start fending for itself was delayed. As an adaptation to this delay a store of food was provided by the mother for consumption by the embryo in the form of yolk in the egg, or provision was made for the continuous supply of nutriment from the mother to the embryo by means of a placenta. By these means the embryonic period was prolonged at the expense of the larval, and structures which had been larval in previous ontogenies would (unless they were delayed also) come to be embryonic. When, therefore, it is desired to speak of a structure or character which appears in early stages of development, it may be either embryonic or larval. Which it actually is does not matter for the present purposes, and so, in order to avoid making a distinction where none is meant, in the following pages the word 'young' will be used merely to express an early

-34-

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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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