Principles of Development

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Principles of Development
Fertilization
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Cleavage
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At the end of cleavage, Polychaete
worms have 1000 cells and this
frog has around 700,000 cells.
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Isolecithal (ī”so-les'i-thul) eggs - very small amount of yolk mammals
Mesolecithal eggs - moderate amount of yolk - typical of
amphibian eggs and also the platypus
Telolecithal eggs - large amount of yolk – typical of birds and
reptiles
Centrolecithal eggs – Nuclei divide without cytokinesis –
yolk in center of egg, cells form at surface
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Isolecithal
Mesolecithal
Telolecithal
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Centrolecithal
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Blastula to Gastrula
Cells differentiate into two or three
germ layers (vertebrates)1 (insects)2:
• future endoderm (gut, liver, lungs)1 (gut) 2
• future mesoderm (skeleton, muscle, kidney, heart,
blood) 1 (muscle, heart, blood) 2
• future ectoderm (skin, nervous system) 1 (cuticle,
nervous system) 2
• In vertebrates, a rod-like structure of mesoderm
(notochord) extends from head to tail
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Cell Fate
•  As cells proliferate and migrate, they become
determined and then differentiate
Modular Development
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Developmental Strategies
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Amniota: Reptiles, Birds, Mammals
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Altricial
Altricial
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Precocious
Precocious
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