Changes

*Formation of the '''neural ectoderm''' is the default pathway.

*Formation of the '''neural ectoderm''' is the default pathway.

**Therefore, in order to make epidermis, a signal from the environment must be diverting cells from going down this default pathway.

**Therefore, in order to make epidermis, a signal from the environment must be diverting cells from going down this default pathway.

**This signal comes from the lateral plate mesoderm (LPM) which directs ectoderm to become epidermis.  

**This signal comes from the lateral plate mesoderm (LPM) which directs ectoderm to become epidermis. This signal is the secretory factor BMP-4.

***The notochord (lying along the midline) secretes an antagonist to the factor of the LPM so that neural ectoderm is formed above the notochord.

***The notochord (lying along the midline) secretes an antagonist, Noggin, to the factor of the LPM (BMP-4) so that neural ectoderm is formed above the notochord.

*Therefore, in lateral regions ectoderm becomes epidermis and along the midline ectoderm becomes neural ectoderm.

*Therefore, in lateral regions ectoderm becomes epidermis and along the midline ectoderm becomes neural ectoderm.