In oviparous species, eggs are encapsulated in a tough, fibrous case. Some authors have suggested that this egg case is impermeable to solutes and therefore protects the embryo ionically and osmotically until it can regulate on its own. However, more recent evidence suggests that the case if highly permeable to ions and urea and that the embryo in bathed in a solution ionically similar to sea water. The embryo must osmoregulate at the time the egg is laid. Yet just several months into the 22 month gestation period the embryos are capable of independent iono- and osmoregulation in a uterine solution resembling sea water.
Data are sparse concerning more advanced viviparous species.
Embryos of Mustelus canis develop in a solution which is ionically and osmotically similar to maternal plasma. Thus, while iono- and osmoregulation in the embryo of this species would appear to be unnecessary, there is no direct either for or against the embryo's ability to regulate. It is clear, however, that in at least some of the most primitive species of elasmobranchs, the ability of the embryo to regulate salts and urea is present at the earliest stage of development. Thus the ability of the embryo to regulate salts and urea would allow the evolution of a diverse array of reproductive strategies in the elasmobranchs. Instead, we observed that unused sperm nuclei quickly disappeared.