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In a limited temperature range above the clearing point of crystal suspensions, submicroscopic entities representing unmeasurably small portions of the original crystal population may survive whose existence is revealed by crystals nucleated on subsequent cooling [l]. Factors influencing the number of these nuclei and the nature of these nuclei themselves were explored in the case of polyethylene.It was found that the number of nuclei a s assessed from the dimensions of the uniform-sized seeded crystals depends on the temperature to which the suspension was heated (Ts), the rate of heating to Ts, the morphology of suspension before dissolution, and the molecular weight distribution. It was not affected by the time the suspension was kept at Ts o r by the dilution of the starting suspension, and only slightly by the subsequent crystallization conditions. trusions within self -seeded crystals. They were also detected and analyzed by light scattering above the clearing point of the suspensions. The picture emerges that they consist of high molecular weight material which has been appropriately stabilized by refolding on heating up. The observed correspondence of mass and number of nuclei with the high end tail of the molecular weight distribution of the starting material is consistent with each nucleus being a monomolecular crystal. Furthermore, the observed temperature depenThe nuclei were identified electromicroscopically a s central pro-* P r e s e n t address: Petrochemical and Polymer Laboratory, Imperial Chemical Industries LM., Runcorn, Cheshire, England.