part of proventriculus proventriculus inner layer FBbt:00005753 fly_anatomy.ontology inner wall of the cardia The inner layer of the proventriculus, produced from the invagination of the posterior region of the esophagus (foregut). It is formed by a simple low cuboidal epithelium. The cuticle the lines the inner wall is thicker than other regions in the cardia, and it forms several longitudinal folds, causing the esophageal lumen to appear irregular in cross section. gut epithelial section foregut epithelium