Sea Stars:
* Blastula: The sea star blastula is a hollow ball of cells. This is the typical blastula structure in many invertebrates.
* Gastrulation: Gastrulation in sea stars occurs by invagination, where a portion of the blastula wall folds inward, forming a pouch that eventually develops into the digestive tract.
Humans:
* Blastula: The human blastula is a solid ball of cells called a morula. It then hollows out to become a blastocyst, which has an outer layer of cells (trophoblast) and an inner cell mass.
* Gastrulation: Gastrulation in humans is more complex and involves a process called epiboly, where cells from the outer layer of the blastocyst spread over and engulf the inner cell mass, forming the three primary germ layers (ectoderm, mesoderm, and endoderm).
Other differences:
* Cleavage: Sea stars have radial cleavage, where the cells divide parallel or perpendicular to the animal-vegetal axis. Humans have spiral cleavage, where the cells divide at an angle to the axis.
* Fate of blastomeres: In sea stars, the blastomeres have determinate cleavage, meaning their fate is determined early on. In humans, they have indeterminate cleavage, meaning they can develop into any cell type.
In summary:
The major difference in the cellular stages lies in the structure of the blastula and the mechanism of gastrulation. This ultimately leads to different developmental patterns and body plans for sea stars and humans.
