Animal Development

Development is the process by which an animal makes its body. On this page you'll see a couple of microscope slides showing the earliest stages of animal development.


  • Compare and contrast fundamental mechanisms of plant and animal development.
  • Recognize the early developmental stages of starfish and relate these to key events in animal development.

Specimens: microscope slides

  • Starfish early cleavage w.m.
  • Starfish ova unfertilized w.m. (on the same tray as the early cleavage slide)
  • Starfish blastula and gastrula w.m. (We have two versions of this slide: w.m. means whole mount, with the whole embryo placed on the slide. Sec. means that the slide contains sections, or slices, of the embryos. Both slides are worth looking at, but the w.m. version is better.)

Animals and plants develop differently

In an earlier lab you looked at plant development. In vascular plants, primary growth occurs at apical meristems, which are groups of cells specialized for cell proliferation. New, undifferentiated cells are produced by meristems in the growing regions of the plant; later, these cells are signaled by hormone gradients to differentiate into the appropriate types of cells. In plant tissues, cells are locked into place by the cell walls that they share with their neighboring cells, so cells don't move relative to one another.

In animal development, cell movements are essential. Animal cells are attached to one another by specific proteins that not only allow for cell recognition and attachment, but also send vital information from one cell to another to cause cells to follow particular paths of differentiation. During embryonic (and later) development, cells move around, make contacts with neighboring cells, and receive the instructions that will determine their developmental fates. These cell movements begin in the blastula and gastrula stages, described below.

Starfish embryos

Starfish, sea urchins, and other members of the phylum Echinodermata are classic model organisms for studying developmental biology. They typically have external fertilization, which makes them easy to observe. The eggs and larvae are reasonably large, but are small enough to fit on a microscope slide. Fertilization is easy to achieve, simply by putting eggs and sperm together in seawater. The earliest stages of embryonic development roughly correspond to those in all other animals, including vertebrates. We have a series of slides showing embryonic development in starfish.


Starfish oocyte, whole mount

This oocyte is a future egg, caught in the process of meiosis. The nuclear envelope (double membranes surrounding the nucleus) is clearly visible, as is the nucleolus (a structure in the nucleus which makes ribosomes). These structures will disappear when meiosis is complete and the egg is fertilized.

The egg is a very large cell. After fertilization, it will begin to divide to form an increasing number of smaller and smaller cells. Only when the young starfish reaches a feeding embryo stage will it begin to take in food and actually become more massive.

Click on the image to shrink/enlarge.

Cleavage stages

After a zygote (fertilized egg) divides, it is called an embryo. In starfish and most other invertebrates, it is called an embryo until it becomes complex enough to swim and feed, at which point it is called a larva.

The early embryonic cell divisions are known as cleavages. These divisions occur in a tightly controlled fashion so that as new cells are produced, they take up specific positions in the embryo.

The left side of this picture shows an embryo in the two-cell stage, after the first cleavage.

The right side of the picture shows a 16-cell embryo, in a stage called the morula. At this stage, the embryo is a more or less round, solid ball of cells. The morula overall is approximately the same size as the egg before fertilization, so at this point each cell contains approximately 1/16 of the mass of the original egg.


As cell divisions continue to occur, the cells of the solid morula move around to form a hollow ball. This embryonic stage is known as the blastula. The interior of the blastula is filled with water; this space is called the blastocoel.

The movements of cells in the embryo is one of the most important characteristics distinguishing animal development from plant development. The cells of an early embryo are undifferentiated, meaning that they all look the same and produce roughly the same set of proteins. As these cells begin to move around during development, they communicate with other cells that they contact. Eventually, this cell-cell contact determines the developmental fate of each cell.

In contrast, plant cells are locked in place by the cell walls that they share with neighboring cells; their developmental fates are determined by varying concentrations of hormones produced nearby parts of the plant.


The blastula goes begins the next important morphogenetic movement, called gastrulation. In gastrulation, the cells of the embryo begin to rearrange themselves so that the hollow-ball blastula forms an indentation. Once this happens, the embryo is called a gastrula.

The indented part surrounds a space, called the archenteron,  that will become the animal's gut. The cells that line the archenteron become the embryonic endoderm. The remaining cells, on the outside of the gastrula, are the embryonic ectoderm. In a later stage, some cells will move into the blastocoel and become the embryonic mesoderm.

This image shows a whole-mount gastrula; if you have this on your microscope, you can focus up and down to reveal the three-dimensional structure.

All animals perform gastrulation, but there is some variation in what the gastrula looks like.


Terms & Concepts

  • Archenteron
  • Blastula
  • Blastocoel
  • Blastopore
  • Embryo
  • Ectoderm
  • Endoderm
  • Gastrula
  • Larva
  • Mesoderm
  • Morula
  • Oocyte

Review Questions

1. What is this structure? (You should be prepared to recognize any of the structures listed above, either in the microscope slides shown on this page or in the frog gastrulation model set.)

2. What happens in gastrulation? Which groups of animals perform gastrulation? Why is gastrulation important in animals but not in plants?

3. Which embryonic tissue layer produced this structure?

  1. Ectoderm
  2. Endoderm
  3. Mesoderm

On a lab exam, I could ask you this question about embryonic tissue layers with reference to the embryo slides, embryo models, tissues, or animal cross sections.

References & further reading

Campbell Biology, Chapter 47

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