Lipids carry out important structural as well as signaling functions in the cell. In recent years, enzymes that metabolize lipids have been emerging as key regulators of basic cellular functions and developmental processes. In order to study metabolism of lipids, we have focused our research on a class of proteins: the cytochrome P450s (CYPs), which are involved in lipid production in many organisms. We have used C. elegans, a classical genetic model system, to investigate lipid metabolism because this nematode offers several technical advantages that render it suitable for our investigations. The aim of our project was to identify and characterize essential lipids for the development of worms. We have performed RNAi (RNA interference) against C. elegans CYP31A, and found that silencing of this enzyme leads to the arrest of embryonic development. Further characterization of this embryonic lethal phenotype revealed that it is caused by problems in establishment of polarity and failure in the extrusion of a polar body. Moreover, we found that embryos depleted of CYP31A are osmotic sensitive and their eggs are permeable to dyes (hoechst, FM 4-64 etc.). The defects described above are common to a class of mutants that received the denomination of POD (for Polarity and Osmotic Defects). Analysis by electron microscopy demonstrated that cyp31A(RNAi) embryos exhibit an improperly constructed eggshell. Further functional studies have demonstrated that the defects observed in cyp31A(RNAi) embryos can be ascribed to the malfunctioning of one of the three layers of the eggshell: the lipid-rich layer, but additional problems in the assembling of the other two layers are also present. In order to identify the product of CYP31A, we set up a bioassay in which we tested the capability of lipidic extract from wild type embryos to rescue the embryonic lethality. The bioassay provided a method to track the activity and allowed us to enrich the metabolic product of CYP31A by the fractionation of the total lipid extract. Another POD gene, emb-8, codes for an NADPH CYP reductase. This 4 protein supplies electrons to the CYPs for their metabolic reactions. A mutant of emb-8 (emb-8(hc69)), gives a similar phenotype as the knockdown CYP31A. With the aim to test if EMB-8 and CYP31A act in the same pathway we extracted lipids from emb-8TS mutants. We tested in the bioassay if extracts from emb-8(hc69) mutants, containing the metabolic product of CYP31A, can rescue cyp31A(RNAi) phenotype. The results obtained suggest that EMB-8 and CYP31A work in the same metabolic pathway. Conclusively, CYP31A and EMB-8 cooperate to produce a class of lipids that are required for the construction of a functional eggshell. A defective eggshell causes failure in polarity establishment, extrusion of the polar bodies, osmotic sensitivity and permeability and eventually it leads to the arrest of the development of C. elegans embryos.