Helminth eggs have resisted analysis by electron microscopy because fixatives, dehydrating agents, and embedding media penetrate these eggs poorly. Slam-freezing at liquid nitrogen temperature followed by freeze-substitution and Spurr's medium embedment provides preservation of the internal structure of the Schistosoma mansoni egg shell, developing miracidium, and perimiracidial structures. The egg shell consists of the three previously described layers (outer microspinous, middle intermediately dense, and inner dense layers) with cribriform pores. A newly described layer (Reynolds' layer) develops subjacent the egg shell and is comprised of densely-packed branching filaments. A single layer of squamous cells (von Lichtenberg's envelope) closely adheres to Reynolds' layer. Between von Lichtenberg's envelope and the embryo is a space (Lehman's lacuna); this space is initially filled with electron-lucent fluid, but subsequently masses of granulofloccular material (Cheever bodies) develop; Cheever bodies are partially membrane bound. Epidermal plates differentiate from superficial cells of the embryonal cell mass, while epidermal ridges differentiate from cells just below the surface of the embryonal cell mass. The cytoplasmic layer (von Lichtenberg's envelope) interposed between the host extracellular fluid and the developing miracidia effect a barrier against a simple passive diffusion; this infers that complex macromolecules, such as schistosomal egg antigen, undergo active, and perhaps selective, transport in or out of the egg.