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Carnegie Stage 8 Introduction

Stage 8 embryos are in the presomite period, have a greatest length of 0.5 to 2 mm and an estimated postfertilization age of approximately 21 to 25 days. The embryonic disc is ovoid or pyriform in shape when viewed from the dorsal aspect. The gastrulation (primitive) pit is present. Neural folds herald the beginning of primary neurulation. A primordial head-fold is not yet evident but a primordial tail-fold is present. A gastrulation (primitive) streak and node and notochordal canal are typically present.

The stage is represented by Carnegie embryo #10157 that was given a grade of excellent. A gastrulation streak and notochordal canal could not be identified. A transverse groove that is likely a fixation artifact is present in the caudal part of the neural plate. The specimen was prepared for microscopic examination in 1967 when it was fixed in formol, embedded in paraffin, and serially sectioned transverse to the long axis. The section thickness was not recorded but it is estimated to be 8 microns. The sections were mounted on 62 small glass slides and stained with cason. There are 225 sections through the embryo and adjacent membranes of which 121 have been digitally restored, labeled, and can be viewed at four magnifications.

The morphology of this embryo has not been documented in the literature. There are no known photographs of the embryo before it was sectioned and the Carnegie collection has no reconstructions of the embryo.

Several 3D reconstructions have been produced from the aligned sections. Animations of 3D-reconstructions of the embryo surface and the internal anatomy together with fly-through animations of the aligned sections are also included on the disks.

Source: The Virtual Human Embryo.