| 1 | In the center layer (mesoderm), the heart and circulatory system are forming. | |
| 2 | Two cell layers( diploblastic) separated by a gelatinous mesoglea. | |
| 3 | Fibroblasts are mesenchymal cells which are derived from the embryonic mesoderm. | |
| 4 | there are many genes that are already expressed and presented in this dimpling and thickening of the outer layer. | |
| 5 | This suggests germ layer boundaries are less absolute than previously thought. |