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. Author manuscript; available in PMC: 2010 Oct 4.
Published in final edited form as: Bioessays. 2009 Aug;31(8):892–902. doi: 10.1002/bies.200900038

Figure 1.

Figure 1

The posterior primitive streak is confined to the embryo proper and gives rise to the allantois. Modified from Sobotta (1911)(10), Figs. 9, 10, and 12, with kind permission of Springer Science + Business Media. Staging of Sobotta's sections was according to Ref. 14 with recent further refinements at the headfold stages in Ref. 8. A: Presumptive late streak (LS) stage (~7.0 dpc) showing the formation of the exocoelom (x) and the extent of the primitive streak (ps) within the embryonic compartment of the conceptus. The nascent amnion (am) is forming. B: Presumptive very early headfold (vEHF) stage (~7.75 dpc) depicts the allantoic bud (al) in association with the intraembryonic primitive streak, whose posterior limit is within the embryo proper, just beneath the amnion. C: Presumptive early headfold (EHF) stage (~7.75–8.0 dpc) shows an enlarging allantois. The primitive streak, black line within the nascent amniotic cavity, traced from Sobotta's original figures, is limited to the embryo proper.