Stage 21 embryos have a greatest length of 20 to 26.4 mm and an estimated post-fertilization age of approximately 49 to 52 days. At this stage the fingers become more elongated and the hands and feet approach each other. For the first time the cortical plate is present in the wall of the cerebral vesicle (hemisphere). The stage is represented by Carnegie embryo # 4090 that has a grade of good and a greatest length of 22.2 mm (after fixation). This female specimen is considered to be early within the stage.

The embryo was prepared for microscopic examination around 1922. It was fixed in Formol, embedded in paraffin and serially sectioned transverse to the long axis at 40 microns. The sections were mounted on 24 large glass slides and stained with alum cochineal (carmine). There are 393 sections through the embryo.

The Browse part of the DREM database includes 195 of the 393 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. Animations of the 3D reconstructions of the embryo surface and fly-through animations of the aligned sections are also included on the disks. For anyone who wishes to use them for other reconstructions, research or presentations, all the original section images are available as individual .jpg files.

Instructions for using the disks can be found by going to the Instructions section from the opening screen.