Final answer:
Reproduction challenges were solved by egg laying with a harder shell by providing protection against environmental hazards and dehydration, while the yolk within the egg offers vital nourishment for the embryo's growth.
Step-by-step explanation:
The advancement of egg laying with a harder shell in animals like birds and reptiles was a significant evolutionary step in reproduction. In species that use internal fertilization, oviparity is a method where fertilized eggs are laid outside the female's body. The eggs of birds and some reptiles like turtles have hard calcium shells. The hard leathery exterior of reptile eggs and the hard calcium covering of bird eggs protect the growing embryo from environmental dangers. Moreover, the shell prevents dehydration by being water tight. The embryo within the egg receives nourishment from the yolk, which is rich in nutrients necessary for development. This method of reproduction allows for the laying of eggs in a variety of environments without the constant need to keep the eggs moist, thus enhancing survival rates.
For example, chicken eggs, which have a high concentration of calcium carbonate in the shell, are well-known for their hardiness. Reptiles, on the other hand, produce eggs that may have either a hard shell or a leatherier consistency. In all these cases, the egg's structure, including its amnion, a fluid-filled membrane, facilitates a safe and protective environment for the embryo until hatching.