Final answer:
The amniotic egg is compared to a spacecraft because it provides a self-contained life support system for the developing embryo, similar to how a spacecraft sustains life in space. The extraembryonic membranes and shell protect and nourish the embryo, allowing vertebrates to inhabit drier environments. Even in mammals, the placenta serves a similar function, supporting and protecting the developing embryo within the mother's body.
Step-by-step explanation:
The textbook author compares the amniotic egg to a spacecraft because it encapsulates a suite of life-support systems similar to how a spacecraft supports life in the harsh conditions of space.
Like a spacecraft, the amniotic egg is designed to protect and nourish life within it, providing a self-contained environment where its occupant can develop and grow.
The amniotic sac and its fluid act like a water-filled balloon, cushioning and protecting the embryo, which allows it to develop in various environmental conditions without the need for an external water source.
The four extraembryonic membranes (amnion, chorion, yolk sac, and allantois) of an amniotic egg have evolutionary significance; they enable less dependence on a watery environment, allowing the embryo to have an aquatic environment while being insulated from the external environment.
This advancement allowed amniotes to explore drier habitats, which was a key evolutionary step distinguishing them from the water-dependent amphibians.
The eggshell, whether hard like that of birds or leathery like that of reptiles, plays a crucial role in water retention and protection, while allowing for essential gas exchange.
In mammals, although the amniotic egg is not laid externally, the placenta and amnion still create a comparable protective environment for embryo development within the mother's body.
The placenta is a result of evolutionary changes in the role of the extraembryonic membranes, now facilitating nutrient transport, gas exchange, and waste management, functioning somewhat like a life support system in space travels.