Sea Star Embryology

Use these images as you study preserved material (slides) of Sea Star development. Find similar stages on your slide. Then record your observations including sketches of each of the stages. Sketches should be included with your laboratory report.

 1. Egg and Zygote The unfertilized egg (1a) has a large and conspicuous nucleus. Compare the amount of yolk and cytoplasm to the size of the nucleus. The nucleus of the zygote (1b) is less conspicuous. The fertilization membrane is not present in this preserved material.
 2. 2 cell stage Successive cell divisions divide the zygote into smaller and more numerous cells. Compare the size of the 2,4 and 8 cell stages with the size of the original zygote. Note the cleavage furrows formed by the pinching in of the cell membranes during division. From the arrangement of the cells determine the plane of cleavage that produced the cells.
 3. 4 cell stage
 4. 8 cell stage
 5 Early blastula Cleavage continues producing many additional cells which arrange themselves around a fluid filled cavity (blastocoel). The blastula (7) begins to take on the appearance of a ball.
 6. Blastula This ball-like stage is called the blastula (5). Compare the size of the blastula to the original zygote. Note the blastocoel (6).
 7. Early gastrula During the late blastula stage a few cells on one side of the developing embryo begin to push into the blastocoel (8). When this process (gastrulation) becomes visible as an indentation, the developing embryo is termed a gastrula (9). The point of indentation is termed the blastopore (10). As the invagination process continues (11),with more cells pushing into the interior, a new cell layer is formed and a new cavity (12) (archenteron) appears within this new layer of cells. This cavity opens through the blastopore (14) to the outside of the embryo. Later a third layer of cells develops between the original two layers. These three cell layers (ectoderm, endoderm and mesoderm) provide the "bricks and mortar" for the differentiation of the embryo's organ systems.
 8. Gastrula
 9. Larval stage The developmental process to the end of the gastrula (13) stage takes approximately two days. At this time the gastrula transforms itself into a free swimming (via cilia bands) (18) and feeding larva (15). This change includes the formation of a digestive system with two openings (mouth (17) and anus (16)). The larva grows as it feeds on algae and diatoms. After several months the larva undergoes a process called metamorphosis that transforms it into a tiny sea star.

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