Germinal Vesicle

During development, the maturing ova are arrested during meiosis I (frequently at diplotene of prophase). Upon contacting sea water, the primary oocytes resume meiosis, the nucleus (germinal vesicle) is degraded, and the first and second polar bodies are formed by 1.5 and 2 hours, respectively. Fertilization can occur anytime during this time period.

  1. Plasma Membrane
  2. Nuclear Membrane
  3. Nucleoplasm
  4. Nucleolus

Unfertilized Egg

The mature ovum of the starfish is approximately 90 to 100 micrometers in diameter and roughly spherical. Notice that the cytoplasm is relatively uniform in appearance, that is, there is a small amount of yolk and it is distributed evenly throughout the egg. Such eggs are called microlecithal.

Zygote

Shortly after fertilization, the vitelline membrane lifts off the egg cell membrane to become the fertilization membran. This functions as permanent block to polyspermy and is a distinguishing difference between fertilized and unfertilized eggs.

  1. Fertilization Membrane

2-cell Stage

During cleavage, microlecithal eggs always divide into complete daughter cells. This pattern of cleavage is termed equal holoblastic. The first cleavage furrow passes through both animal and vegetal poles, and cuts the egg into two equal sized blastomeres (cells).

  1. Fertilization Membrane
  2. Blastomeres

4-cell Stage

Both starfish and sea urchins exhibit radial cleavage (cleavage plains are parallel or at right angles to each other). As seen here, the second cleavage is meridional, but at a right angle to the first.

  1. Fertilization Membrane
  2. Blastomeres

8-cell Stage

The third pair of cleavages are horizontal and produce eight blastomeres. All cleavages are equal and the blastomeres are loosely arranged rather than in tiers of cells.

  1. Fertilization Membrane
  2. Blastomeres

Morula

During the early period of cleavage, as in the 16- and 32-celled stages, the blastomeres are packed together like a cluster of mulberries, and the embryo is referred to as the morula.

  1. Fertilization Membrane
  2. Blastomeres

Blastula

As cleavage continues a fluid filled blastocoel forms and the blastomeres become displaced into an organized peripheral monolayer of cells; the embryo is now referred to as a blastula.

  1. Blastocoel
  2. Fertilization Membrane
  3. Blastoderm

Early Gastrula

In echinoderms, the first sign of gastrulation (formation of the three germ layers) is the flattening of the blastula at the vegetal pole. This flattened region then gradually folds inward, converting the spherical blastula into a cup-shaped gastrula.

  1. Blastocoel
  2. Blastoderm
  3. Blastopore
  4. Archenteron

Mid Gastrula

During gastrulation, the blastocoel is displaced and a new cavity, the archenteron, is formed. The opening into the archenteron is the blastopore and develops into the anus in all deuterostomes.

  1. Blastocoel
  2. Ectoderm
  3. Blastopore
  4. Archenteron
  5. Endodermal Wall of Archenteron
  6. Archenteric Vesicle

Late Gastrula

By the late gastrula stage all three germ layers are visible with ectoderm forming the outside surface, endoderm lining the archenteron, and enterocoelic mesodermal pouches located on either side of the archenteron.

  1. Oral Lobe
  2. Ectoderm
  3. Anus
  4. Coelomic Sacs
  5. Presumptive Esophagus
  6. Presumptive Stomach

Early Bipinarria Larva

The early bipinnaria larva of an echinoderm is little more than a modified gastrula. Using the high magnification image, notice that the blastopore has become the anus and a new opening, the mouth, has appeared in the body wall and becomes continuous with the archenteron to complete the digestive tract.

  1. Oral Lobe
  2. Esophagus
  3. Coelomic Sacs
  4. Stomach
  5. Anus
  6. Anal Lobe
  7. Mouth

Late Bipinarria Larva

In this older larva the digestive tract and bands of cilia are well developed. Note that the larva is still bilaterally symmetrical. In later stages of development the larva will become radially (or pentaradially) symmetrical.

  1. Ciliated Band
  2. Mouth
  3. Anus
  4. Esophagus
  5. Coelomic Sac
  6. Stomach