Final answer:
The combination of a hard outer shell and a soft inner lining in eggs provides protection and a stable environment for the growing embryo. Bird eggs have a hard calcium covering, while reptile eggs have a leathery exterior. The yolk provides nourishment for the embryo's development.
Step-by-step explanation:
Some cases, particularly the eggs of different species, are designed with a soft inner lining and a hard outer shell to provide optimal protection and nourishment to the developing embryo inside. This design is evident in the natural world, where the hard calcium covering of bird eggs protects the embryo from external threats while allowing for the exchange of gases, and the leathery exterior of reptile eggs provides similar protection with more flexibility to the shell.
The inner soft lining, in both cases, serves as a cushion and provides a stable environment for the embryo. Additionally, it is the yolk within these eggs that supplies the necessary nourishment for the growth of the embryo, rather than the endosperm or placenta, as birds and reptiles do not have placentas.