Reserial Bud. Embry double layer (epith + perit.) undiff wall. Calvet thought periderm derived from epithelium but G. thinks 2 are separate w. differentiation place in each. Check undiff cells at anterior end of zoecium this!! Differentiated cells at prox. (OK, when you reason this out) Generalization about epithelial cells in organizing Embryonic epithelial cell has pole & a basal {ciliation, secretion}ial membrane cell may be ciliated or secretive or both. epithelial cell basal membrane pole any have no basal membrane secretion pole (wh. secretes cuticle - mixt. of mucin & pectin of mucopolysaccharides secretes mixt. mucin & pectin then cell becomes cuboidal & mucin becomes dominant & calcif. begins after pectinisation Williams.