part of immediately_preceded_by life cycle stage Theiler stage 12 Theiler stage 12a Theiler stage 12b MmusDv:0000109 Theiler developmental stage 12 defined by unturned embryo with first appearance of somite pairs 5-7 and during which neural closure occurs in the region of the 4th and 5th somites, extending in both directions from this site. Headfolds are particularly prominent. Optic placodes are first evident and become indented to form the optic pits. The heart rudiment develops rapidly. The allantois contacts the chorion at the end of this stage. Embryonic age 8 dpc (day post coitum). Refers to human CarnegieStage09. mouse_developmental_stage TS12b