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Carnegie Stage 12 Introduction

Stage 12 embryos have a greatest length of 3 to 5.4 mm and an estimated postfertilization age of 29 to 31 days. There are 21 to 29 body segments (somites) along the body axis. In the older specimens the caudal neuropore is closed and secondary neuralation has begun. Most of the cephalic neural crests can be identified. The otic vesicle has formed but may remain connected to the overlying ectoderm. The sinu-atrial openings and atrioventricular canal are present in the heart. Tabeculation has begun in the ventricular walls. The aortic arches are present and the dorsal aortae are fused. Four pharyngeal arches, grooves and pouches are present. Future hepatic sinusoids are occupied by hematopoetic tissue. The mesonephric duct terminates caudally in the cloaca.

The stage is represented by Carnegie embryo #8943 that has a grade of excellent. It has a greatest length of 3.9 mm (after fixation) and an estimated postfertilization age of 29 days. There are 23 segments (somites) along the body axis, which places it in the early part of the stage.

The embryo was prepared for microscopic examination in 1934 and was originally part of the collection at the University of Chicago (No. H 1481). The specimen was fixed in Zenker's formol, embedded in celloidin and paraffin and serially sectioned transverse to the long axis at 8 microns. The sections were mounted on eight large glass slides (click for images) and stained with hematoxylin and eosin. There are 439 sections through the embryo.

The DREM database includes 146 of the 439 sections. Approximately every third section was digitally restored and labeled, and can be viewed at four magnifications. Several 3D reconstructions were produced from the aligned section images. Animations of the 3D reconstructions of the embryo surface and the embryonic heart together with a fly-through animation of the aligned section images are also included on the disks.

There is very little information available about this embryo. It has rarely been the subject of studies or reconstructions.

Source: The Virtual Human Embryo.