ABSTRACTThe investigation concerns a first attempt to unravel biochemical patterns of significance for morphogenesis inS. commune. The results derive from two different stocks: one with wild morphology, the other exhibiting an impairment of pileus formation. Some morphological and genetical features of the stocks are discussed.A special culture technique allowed fair synchronization and control of development via the medium. This technique is based on the fact that differentiation can be prevented by cultivation in a thiamine‐free medium.Measured changes in protein, RNA, DNA, and soluble cell constituents during thiamine‐induced primordia formation yielded some information concerning the first steps of differentiation.Primordial growth was found to depend upon transport of nitrogenous compounds from the mycelium and the presence of a carbon source in the medium. Most of the assimilated glucose is deposited as polysaccharides in the cell walls of the primordia.Chemical analysis of the cell walls demonstrated that they are mainly composed of two different glucans, called here S‐ and R‐glucan, some properties of which were investigated.Pileus formation was shown to occur without the uptake of nutrients from the medium and to depend upon the withdrawal of substances from stunted fructifications and stroma in particular. Comparison of the two investigated stocks provided strong support for a causal relationship between degradation of the cell‐wall glucans and pileus formation.An analysis was made of respiration and fermentation during development; some findings probably bearing upon the problem of morphogenesis are