An egg goes through a series of mysterious processes as it readies for encountering sperm. In frogs, as the egg passes through the oviduct, it is coated with a series of layers – rich glycoconjugates and proteins – that comprise the jelly coat of the egg. The structure and importance of the egg jelly has long been known – in Xenopus, it comprises three distinct layers, and removal of the jelly renders the egg unfertilizable. Recently, two laboratories have begun characterizing the jelly factors that are required for fertilization.As a first step towards identifying the jelly molecules involved in fertilization, Mozingo and Hedrick1xDistribution of lectin binding sites in Xenopus laevis egg jelly. Mozingo, N.M. and Hedrick, J.L. Dev. Biol. 1999; 210: 428–439CrossRef | PubMed | Scopus (9)See all References1 provided a detailed description of the distribution of the sugar moieties comprising much of the structural components of Xenopus egg jelly. The differences in sugar distribution and abundance in each jelly sublayer was determined by lectin binding. Preincubating eggs with some lectins inhibited sperm penetration of the jelly, suggesting that the sugar moieties might be involved in sperm binding.By contrast, Olson and Chandler2xXenopus laevis egg jelly contains small proteins that are essential to fertilization. Olson, J.H. and Chandler, D.E. Dev. Biol. 1999; 210: 401–410CrossRef | PubMed | Scopus (27)See all References2 report that proteins that can diffuse from the jelly are both necessary and sufficient for restoring sperm binding to jellyless eggs. Significantly, neither the structural components nor a range of charged polymers could restore fertilization to jellyless eggs. The diffusible factor must be present at the time of fertilization as pretreating jellyless eggs with the diffusible fraction and then removing it prior to sperm addition did not promote fertilization. The fertilization-restoration activity is probably a protein as it is destroyed by heat and protease treatment and is small – 60% of the activity is able to pass through a filter possessing a 50 kDa cut off. Although candidates for this diffusible factor have not been reported, once identified, it will be important to understand the relationships between the protein and glycoconjugate ingredients of the egg jelly in order to reconcile the interesting differences between these two studies.