Fig. 7. Model of restriction of size and patterning of animal plate. (A) The egg has a neuralizing potential in the animal half. The size and patterning of the neural tissue depends on the ß-catenin-mediated specification of endomesoderm. (B) During cleavage, specification of endomesoderm in vegetal blastomeres also specifies ectoderm as ciliary band, which restricts neurogenic ectoderm to a small animal plate. (C) As the ectodermal domain is specified as ciliary band, oral and aboral ectoderm, neuron formation is patterned in the animal plate. Nodal acts as a paracrine repressor of serotonergic neuron formation where oral ectoderm is adjacent to the animal plate. Serotonergic neurons form where the animal plate is adjacent to ciliary band and aboral ectoderm. Red, animal plate; blue, endomesoderm; purple, ciliary band; yellow, oral ectoderm; light green, aboral ectoderm; green cells, serotonergic neurons.