The ectoderm and the endoderm are separated by an extra-cellular matrix layer called the mesoglea. Cnidaria begin to differentiate their nervous systems in the late gastrula.[3] In Hydrozoa and Anthozoa, interstitial stem cells from the endoderm generate neuroblasts and nematoblasts which migrate to the ectoderm and provide for the formation of the nervous system along the anterior-posterior axis. Non-hydrozoa lack interstitial stem cells, and the neurons arise from epithelial cells, which are most likely differentiated from the ectoderm as occurs in vertebrates.