Stages Of Development Of Sea Stars 2j5u71

Germinal Vesicle During development, the maturing ova are arrested during meiosis I (frequently at diplotene of prophase). Upon ing 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 to 2 hours, respectively. Fertilization can occur anytime during this period. 1. 2. 3. 4.

Plasma Membrane Nuclear Membrane Nucleoplasm 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 membrane. 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 holoblastic. The first cleavage furrow es 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 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 32celled 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 the 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 gradually folds inward, converting the spherical blastula into a cupshaped gastrula. 1. 2. 3. 4.

Blastocoel Blastoderm Blastopore Archenteron

Mid Gastrula During gastrulation, the blastocoel is displaced and a new cavity, the archenteron, is formed. The opening in the archenteron is the blastopore and develops into the anus in all deuterostomes. 1. 2. 3. 4. 5. 6.

Blastocoel Ectoderm Blastopore Archenteron Endodermal Wall of Archenteron 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, the enterocoelic mesodermal pouches located on either side of the archenteron. 1. 2. 3. 4. 5. 6.

Oral Lobe Ectoderm Anus Coelomic Sacs Presumptive Esophagus Presumptive Stomach

Early Bipinarria Larva The early bipinarria larva of an echinoderm is a 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. 2. 3. 4. 5. 6. 7.

Oral Lobe Esophagus Coelomic Sacs Stomach Anus Anal Lobe Mouth

Late Bipinarria Larva In this older larva, the digestive tract and the bands of the 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. 2. 3. 4. 5. 6.

Ciliated Band Mouth Anus Esophagus Coelomic Sac Stomach