Laboratory Notes for BIO 1003

© 30 August 1999, John H. Wahlert & Mary Jean Holland

Domain Eukarya
Kingdom Animalia


The slide of stages in starfish development illustrates stages from unfertilized egg to gastrula—typical of echinoderms and chordates that have very little yolk in the egg. Three embryonic cell layers—ectoderm, endoderm, and mesoderm—are established. All adult tissues are derived from these layers.

Starfish development w.m. (no high power). This slide shows the early developmental stages from single cell through early gastrula. Division of a zygote is called cleavage, which is mitosis.

Identifiable stages are:

  • ovum, an unfertilized egg; nucleus and nucleolus are visible
  • zygote, a fertilized egg (diploid); the nucleus is not visible
  • 2 cells, the product of mitosis of the zygote, cells size is halved
  • 4 cells

[development stages]

  • 8 cells
  • morula (like a compact cluster of grapes)
  • blastula, a hollow ball of cells—early, middle, and late stages are determined by cell size, which gets smaller with each round of mitosis

[development stages]

  • gastrula in which the surface pushes into the interior at a point called the blastopore to form a tube that will become the digestive system. The blastopore is the future anus of the starfish. In a gastrula the cells on the outside are ectoderm, those lining the inner tube are endoderm, and cells that migrate and multiply between these layers will become the mesoderm.

[development stages]

  • larva—some slides show a further developmental stage. The larva is a ciliated form in which you can see a mouth, digestive system, and anus. It will metamorphose into an adult starfish.


Note that all structures are hardly bigger than the original unfertilized egg; the cells produced by mitosis have not grown. How does the size of an egg cell compare with that of an average somatic (body) cell?

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Last updated 22 September 2007 (JHW)