When a solution of interpenetrating and entangled long flexible polymer chains is cooled to low enough temperatures, the chains crystallize into thin lamellae of nanoscopic thickness and microscopic lateral dimensions. Depending on the nature of the solvent and growth conditions, the lamellae exhibit several sectors that have differing growth kinetics and melting temperatures. Remarkably, these lamellae can spontaneously form tentlike morphology. The experimentally well-documented phenomenology of lamellar sectorization and tent formation has so far eluded a fundamental understanding of their origins. We present a theoretical model to explain this longstanding challenge and derive conditions for the relative stabilities of planar, sectored, and tent morphologies for polymer lamellae in terms of their elastic constants and interfacial tensions. While the present model offers an explanation of the origin of the spontaneous formation of sectored tentlike morphology as well as sectored planar morphology, in contrast to planar unsectored morphology, predictions are made for morphology transformations based on the materials properties of the polymeric lamellae.