Blastomere which lies adjacent to the prospective ventral midline in the first tier of blastomeres. It contributes to epidermal and placodal derivatives including the epidermis, lens, cement gland, cranial ganglia, otic vesicle, neural crest, Rohon-Beard neurons, and spinal cord (but no other CNS structures). It also contributes to the pharyngeal mesoderm, heart, somites, pronephric kidney, lateral plate mesoderm and tail fin mesenchymes, and a few cells in the gut.