The amniotic egg

The first and most major thing that separates fish and amphibians from all other vertebrates is the amniotic egg. The first question that comes to mind, though, is “what is an amniotic egg?”

The amniotic egg is an egg with a shell, literally. However, it gets a bit more complicated than that. Mammals are considered amniotes, even though only the platypus and echidna actually lay eggs today. All amniotes are defined by two characteristics: the eggs don’t have to be laid in water, and eggs have a protective membrane called the amnion. In reptiles, birds, and egg-laying mammals, the amnion lies under the eggshell, which is hard and brittle in birds, but soft and leathery in reptiles and mammals.

The amnion surrounds the embryo during development, and the common term ‘the water’s breaking’ refers to the amnion rupturing and the fluid inside flowing out. Cats and dogs are born in the amnion; their mothers rupture it and eat the amnion. The amnion, coupled with yolk and various other membranes are the main component of an amniotic egg.

The reason this is so important is because in amphibians, while they can live on land, they always have to find water nearby to lay their eggs. This holds amphibians back from completely filling ecological niches within land habitats. Even when laid in water, amphibian and fish eggs are almost completely exposed to environmental harm and predators. In amphibians, a bitter-tasting jelly deters many predators, but still chemical changes in water can lead to calamity.

An eggshell or a protective sac carried within the mother is much safer, and yolk enables embryos to get more nutrients and protein. Because of this, during the Permian when the temperate swamps and lowland forests dried up, there is a vast drop in amphibian fossils and a huge increase in the number and variety of reptiles. Another feature that amniotes have is skin that can retain water in, a reason that amphibians without such skin have to go find water every once in a while.

That’s the amniotic shell and a basic overview of amniotes for you!