The problem of spherulitic crystallization has been addressed using the system poly 4(methylpentene-1) crystallized isothermally from the melt. Its internal lamellar microstructure has been studied by electron microscopy following permanganic etching for a wide range of crystallization temperatures. In all cases growth proceeds by an iteration of branching and divergence of individual dominant lamellae to give a framework within which subsequent subsidiary growth occurs. Continuation eventually gives spherulites which grow along a common radial direction (110) within a spherical envelope. The essential origin of spherulitic growth is identified as the tendency for adjacent dominant lamellae to diverge from spatially distributed branch points. A principal cause of this is suggested to be the pressure of uncrystallized molecular cilia attached to fold surfaces but lamellae are also shown to respond to changes in the direction of material flux by bending or by utilizing distortions which are present even in isothermally grown lamellae. The growth of lamellae as individuals rather than contiguous groups promotes molecular chainfolding and leads to variation in physical properties most prominently on the scale of the inter-dominant separation.