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Carnegie Stage 20 Introduction

Stage 20 embryos have a greatest length of 18 to 22 mm and an estimated post-fertilization age of approximately 49 days. At this stage the upper limb is longer and slightly bent at the elbow. The hands are still far apart and the fingers are short, stubby and slightly curved over the cardiac prominence. The interorbital groove is conspicuous. The stage is represented in this database by Carnegie embryo #462. This embryo has a greatest length of 20 mm (after fixation) and is considered to be within the most advanced third of the stage. It has been given a grade of excellent and is considered to be the one of the best transversely sectioned specimens in the Carnegie collection at this stage.

This male specimen was prepared for microscopic examination around 1910. It was fixed in formol, embedded in paraffin and serially sectioned transverse to the long axis at 40 microns. The sections were mounted on 39 large glass slides and stained with alum cochineal (carmine). There are 397 sections through the embryo.

The Browse part of the DREM database includes 200 of the 397 sections. Approximately every other section was digitally restored and labeled, and can be viewed at four magnifications. Several 3D reconstructions were produced from the aligned sections, and animations of these reconstructions together with fly-through animations of the aligned sections are also included on the disks.

Source: The Virtual Human Embryo.