内侧鼻隆起 窝 外侧鼻隆走 nasal placode: by the 5th week, thickening of ectoderm of lower part of frontonasal prominence ---nasal pit: depressions in nasal placode median nasal prominence and lateral nasal prominence: proliferation of mesenchyme at the margins of the nasal placodes---nasal placode: by the 5 th week, thickening of ectoderm of lower part of frontonasal prominence ---nasal pit: depressions in nasal placode ---median nasal prominence and lateral nasal prominence: proliferation of mesenchyme at the margins of the nasal placodes