Sea urchin blastula (IMAGE) Indiana University Caption Sea urchins and most other animals begin their embryonic development as small clusters of cells, called blastulas. The blastulas are surrounded by a protective fertilization envelope Credit Rudolf and Elizabeth Raff Usage Restrictions Must include the credit License Licensed content Disclaimer: AAAS and EurekAlert! are not responsible for the accuracy of news releases posted to EurekAlert! by contributing institutions or for the use of any information through the EurekAlert system.