Complex sentence: At about the 10th week of embryonic development the pouch forms by adhesion of the pharyngeal ectoderm to the cranial end of the notochord become closed at the orifice or crusts adhere to the orifice without closing.
Split into simpler sentences: At about the 10th week of embryonic development, the pharyngeal pouch forms by adhesion of the pharyngeal ectoderm to the cranial end of the notochord. This become closed at the orifice or crusts adhere to the orifice without closing.