The development of cleidoic egg in the amniota?

Answer 1

The amniotic egg was an evolutionary invention that allowed the firstreptiles to colonize dry land more than 300 million years ago. Fishes andamphibians must lay their eggs in water and therefore cannot live far fromwater. But thanks to the amniotic egg, reptiles can lay their eggs nearlyanywhere on dry land.

The amniotic egg of reptiles and birds is surrounded by a tough outershell that protects the egg from predators, pathogens , damage, and drying. Oxygen passes through tiny pores in the shell, sothe embryo doesn't suffocate. Inside the shell are four sacs. Thefirst sac inside the shell is the chorion, which carries oxygen from theshell to the embryo and waste carbon dioxide from the embryo to the shell.Within the chorion is theamnion, the membrane for which the amniotic egg is named. The amnionkeeps the embryo from drying out, so it's critical to living onland. A third sac, the allantois, stores wastes from the embryo and alsofuses with the chorion to form the chorioallantoic membrane, which carriesoxygen and carbon dioxide to and from the embryo, just like a lung. Afourth membrane, the yolk sac, holds and digests nutritious yolk for thedeveloping embryo.

Together, the shell and membranes create a safe watery environment inwhich an embryo can develop from a few cells to an animal with eyes andears, brain, and heart. Because reptiles, birds, and mammals all haveamniotic eggs, they are called amniotes.

The duck-billed platypus and some other mammals also lay eggs. But mostmammals have evolved amniotic eggs that develop inside the mother'swomb, or uterus, and so lack a shell. In humans and other mammals, thechorion fuses with the lining of the mother's uterus to form anorgan called the placenta. The placenta transports oxygen and carbondioxide to and from the embryo and delivers nutrients from themother's blood