I
STAGES OF DEVELOPMENT AND STAGES OF EVOLUTION

EMBRYOS undergo development; ancestors have undergone evolution, but in their day they also were the products of development. Out first task must therefore be to define these two sets of events to which living things are subject. We shall concern ourselves mostly with animals, but what we shall have to say about them is, in general, applicable also to plants.


Stages of development and the scale of beings

The life of an animal may be said to start from the egg which has just been fertilized. The egg is a more or less spherical object and it bears no resemblance to the animal that laid it, or into which it will develop. The processes of development are therefore concerned with the transformation of the egg into that form which we recognize as 'the animal'. The transformation is gradual, and so the shape of the spherical egg is modified little by little into another shape, and this is changed into yet different ones which succeed one another until the definitive or adult shape is reached. During its development, therefore, the animal passes through a series of successive stages, and it is the sequence of these stages which we mean when we speak of the animal's ontogeny. Some of the ontogenetic stages are familiar to every one, such, for example, as the caterpillar of the moth or the tadpole of the frog. But, on the whole, the stages of development of animals are more or less unknown to the layman, and he must rely on the embryologist, whose business it is to collect and study these interesting phases of an animal's existence.

Another series of stages can be made out from animals by comparing the full-grown or adult shapes of one kind of animal with those of other kinds, and finding that they can be arranged in an order of increasing or decreasing complexity. Such an order was not unknown to the Greeks, who called it the scale of beings. At the top of Aristotle's scale was man, below whom came whales, viviparous quadrupeds (other mammals), oviparous quadrupeds

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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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