In this thesis I describe the construction of a cDNA library of maternal mRNA from the sea-urchin <i>Arbacia punctulata</i> and the subsequent isolation of a cDNA clone for the cylin protein using the technique of hybrid-arrest of translation. The cyclin protein was originally identified in sea-urchin embryos as a protein that was strongly synthesised after fertilisation but destroyed at each cell division (Evans <i>et al</i>, 1983). The DNA sequence of the putative cyclin clone was determined. It contains an open reading frame for a protein of M<SUB>r</SUB>46,000 which bears significant homology to the central region of the sequence of two other cyclin mRNAs found in eggs of the clam <i>Spissula solidissima</i>. The cyclin clone was subcloned into a T7 RNA polymerase transcription vector and the <i>in vitro</i> translation product of the encoded mRNA found to be a protein of apparent M<SUB>r</SUB>56,000 on a 1-D SDS acrylamide gel, which co-migrated with the <i>in vivo</i> synthesised cyclin protein. When the <i>in vitro</i> transcript was micro-injected into <i>Xenopus Aevis</i> oocytes it caused germinal vesicle breakdown, indicative of entry into meiosis, suggesting a role for cyclin in the control of the cell-cycle.