| When the blastula is oval and free-swimming the inner mass is formed by unipolar immigration from the hinder pole. |
| At this stage, the blastula is in the form of a hollow sphere with radial symmetry. |
| They migrate along the inner face of the blastula wall to form a ring in the vegetal region and lay down calcareous skeletal rods. |
| A lot of research has gone into answering that, and determining the earliest stage at which the blastula breaks symmetry. |
| The blastula differentiates itself into embryonic layers, the ectoderm, endoderm, and mesoderm already mentioned. |
| The round cluster becomes filled with fluid, and we have a hollow sphere of cells, which I call the blastula. |
