Egg envelopes in vertebrates.
As the material presented in this chapter was being collated, our existing perceptions about the basic similarities of vertebrate (and indeed most, if not all, invertebrate) egg envelopes became increasingly strengthened. Perhaps without exception, all vertebrate and invertebrate eggs acquire a "vitelline" envelope. Interestingly, its filamentous ultrastructure and chemical composition--basically protein and carbohydrate--is similar in all species as is its permeability to large molecules. Furthermore, many (if not all) of its functions are shared among the animal phyla as is its potential to become altered at the time of fertilization and, in its altered state, to provide a new set of modi operandi. It provides sperm receptors that are generally species specific and helps prevent polyspermy; it protects the developing embryo yet yields at the time of hatching. In most vertebrate eggs (including some mammals), a jelly or albumen coat is added to the vitelline envelope. These components may vary immensely in thickness, but again their basic chemical composition is common to all. The functions of these envelopes, while perhaps somewhat less clear than those of the vitelline envelope, are related to species-specific fertilization and to embryonic protection. Albumen serves a nutritional role--most clearly shown in the birds. Finally, the shell membrane and shell present in diverse groups contribute additional adaptations for embryo protection. Vertebrate egg envelopes, then, are basically similar; the modifications, including the addition of shell membranes and shells in some groups, reflect adaptations to differing reproductive strategies and to the environmental exigencies with which the egg must cope. With the growth of our understanding about the structure, chemistry, function, and evolution of egg envelopes new questions will continually be formulated. Many will be the same as those asked years ago but they will be answered with newer techniques and with greater insight.