An integrated process and corresponding apparatus that produces a relatively clean, delignified cellulose product from lignocellulosic biomass. The method includes treating a portion of the delignified cellulose itself as a substrate to produce on-site cellulolytic enzymes, including further treating the remaining delignified cellulose with the resulting cellulolytic enzymes for in situ enzymatic hydrolysis. The process and apparatus are useful to produce fermentable sugars for cost-effective manufacturing of fermentable sugars, fuels, bioproducts and chemicals.