| Word | Definitions | Synonyms |
|---|---|---|
| gastrula | (noun) double-walled stage of the embryo resulting from invagination of the blastula; the outer layer of cells is the ectoderm and the inner layer differentiates into the mesoderm and endoderm | - |
| morula | (noun) a solid mass of blastomeres that forms when the zygote splits; develops into the blastula | - |
| spirula | (noun) a small tropical cephalopod of the genus Spirula having prominent eyes and short arms and a many-chambered shell coiled in a flat spiral | Synonyms: Spirula peronii |