Structure of Blastula

The structure of blastula turns into modified in several animal groups. The changes are related to the amount of yolk deposited in the egg, as you will see from the accounts of blastula structure in some deuterostome animals. The blastula of the echinoderm sea cucumbers contains a fluid filled blastocoel surrounded via a single layer of cuboidal blastomeres that constitute the simple epithelial blastoderm. In the blastulae of sea urchin and Amphioxus, the blastoderm surrounding the blastocoel is an epithelium containing a single layer of columnar cells (blastomeres). Though, the vegetal blastomeres are larger than animal blastomere that is why the epithelium is thicker at the vegetal pole and thinner at the animal pole. So the polarity of the egg persists in the blastula.