2002, Rovert E. A. Stewart, Barbara E. Stewart, Female Reproductive Systems, entry in Encyclopedia of Marine Mammals, page 425,
The allantois fuses with the chorion, forming a small round area, the allentochorion. This region becomes the placenta (Fig. 3).
Source: wiktionary
In this illustration, some license is taken, since the amnion is shown as still separate from the chorion; this is done to illustrate the process of apposition of the amniotic sac to the chorion and to accentuate the interposition of the yolk sac between the amnion and the chorion.
Source: wiktionary
The amnion and chorion begin to develop soon after fertilization and continue to grow until about 28 weeks′ gestation.
Source: wiktionary
The chorion has a predominantly lamellar structure, the lamellae being two-dimensional networks of protein fibrils.
Source: wiktionary
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