The blastula then develops a pore or a hole at one end, called a blastopore. Initially, at least four cell divisions occur, giving rise to sixteen cells, collectively referred to as the morula.

The blastula develops a pore at one end, called a blastopore. If that blastopore becomes the mouth of the animal, the animal is a protostome, and if it forms an anus, the animal is a deuterostome.

Robert Briggs, Thomas J. King, Transplantation of Living Nuclei from Blastula Cells into Enucleated Frogs' Eggs, Proceedings of the National Academy of Sciences of the United States of America, Vol.