The Amniotic Egg

John Merck


In a way, claiming that these are seven separate characters seems dishonest since all but the last two are associated with one important life-style change: All amniotes reproduce using an air-breathing egg or its derivative.

Lissamphibian egg from To breathe or not to breathe blog
This is huge. The egg of a lissamphibian is relatively small and simple. The embryo develops inside the perivitelline space enclosed by a Vitelline capsule, outside of which is a thick jelly capsule. Gas exchange is a limiting factor. Gasses enter and leave the egg by simple diffusion. Complicating this, the eggs are often laid in a gelatinous mass where diffusion through the egg mass is slow. Embryos have comparatively low metabolic rates and must hatch at an early developmental stage so as not to asphyxiate.

Amniotic egg from Biology Pictures
Major features of the amniotic egg:
Possible advantages of the amniotic egg:

Caecilian broods eggs from British Museum of Natural History
The odd reproductive strategies of some frogs reflect that they, too experience these selective pressures. Some colorful examples:

But these strategies require very specific environments and/or make heavy demands of parents. The amniotic egg, while metabolically expensive, can be laid and forgotten. Scenarios of it origin are, necessarily, speculative, but the general consensus is that its precursor would have been:

The evolution of the egg, the mechanism of internal fertilization, and the extraembryonic membranes must have proceeded in lockstep. Indeed, oviparous caecilians seem to have convergently (?) evolved something like an amniotic egg precursor.

Additional reading: