Étude comparative de la membrane coquillère chez les souches ovipare et vivipare du lézard Lacerta vivipara

Evolution of viviparity in reptiles has resulted in more or less complete regression of the eggshell membrane. Such a regression has been studied in a lizard, Lacerta vivipara, which has both oviparous and viviparous populations. In oviparous reproduction, eggs laid have parchmentlike eggshells with a mean thickness of 36 μm. These eggshell membranes are composed of fibrils and of calcite, which is distributed over the outer surface and in the interfibrillar matrix. In viviparous reproduction, a transparent eggshell membrane remains between the embryonic and maternal tissues throughout pregnancy. This membrane consists mainly of fibrils and has only minor traces of calcite. Its mean thickness is only 9 μm. Reduction of thickness and of calcification is thought to be an adaptation that allows better respiratory exchanges at the end of pregnancy, when embryos require more oxygen. The author emphasizes that species with bimodality of reproduction (oviparity and viviparity) are of considerable interest in research investigating the evolution of viviparity in reptiles.