The eggs are generally surrounded by the extraembryonic membranes but do not develop a shell, hard or soft, around these membranes.
2.
The hypoblast contributes to extraembryonic membranes and the epiblast will give rise to the ultimate embryo proper as well as some extraembryonic tissues.
3.
Further development of the design, like extraembryonic membranes ( amnion, chorion, and allantois ) and a calcified shell, were not essential and probably evolved later.
4.
In some cases, the vegetal pole is thought to differentiate into the extraembryonic membranes that protect and nourish the developing embryo, such as the placenta in mammals and the chorion in birds.
5.
The inner cells are pushed to one side of the cavity ( because the embryo isn't getting any bigger ) to form the inner cell mass ( ICM ) and will give rise to the embryo and some extraembryonic membranes.
6.
It has been suggested that shelled terrestrial eggs without extraembryonic membranes could still not have been bigger than about 1 cm ( 0.4 inch ) in diameter because of diffusion problems, like the inability to get rid of carbon dioxide if the egg was larger.
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