A structure created during embryogenesis when the lateral mesoderm splits into two layers - the outer (or somatic) layer becomes applied to the inner surface of the ectoderm, and with it forms the somatopleure.[WP].
Comment:
[development-note] "Gives rise to the paired appendages in jawed vertebrates. a persistent somatopleure through the redistribution and expansion of the lateral plate mesoderm is a synapomorphy for gnathostomes and a critical step in generating a novel developmental module with dramatic evolutionary potentia (Tulenko et al. 2012) http://burkelab.research.wesleyan.edu/current-projects/lamprey/"