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FIG 7. Notochordal canalization, intercalation and excalation.
A, The notochordal process contains a central lumen (the notochordal canal), which is continuous with the amniotic cavity through the primitive pit.
B, During intercalation, the canalized notochordal process fuses with the underlying endoderm; the communication of the amnion with the yolk sac forms the primitive neurenteric canal.
C, During excalation, the notochord rolls up and separates from the endoderm to become the definitive notochord; the primitive neurenteric canal becomes obliterated.
Reproduced from (22) with permission of S. Karger AG, Basel, Switzerland.
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