what is the blastula
Cleavage-Blastula Structure. The blastula is a hollow ball of cells organized into an epithelial monolayer. Midway through the blastula stage, the embryo secretes hatching enzyme, which results in proteolytic degradation of the fertilization envelope.leavage-Blastula Structure. The blastula is a hollow ball of cells organized into an epithelial monolayer. Midway through the blastula stage, the embryo secretes hatching enzyme, which results in proteolytic degradation of the fertilization envelope.