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Carnegie Stage 6 Introduction

Stage 6 embryos are in the presomite period and have an estimated postfertilization age of 16 to 21 days and a greatest length of approximately 0.3 mm. When the gastrulation (primitive) streak is present in the caudal part of the specimen it is sometimes classified as Stage 6b. Similar specimens without a gastrulation streak are then classified as Stage 6a. Chorionic villi and the amniotic cavity and secondary umbilical vesicle (yolk sac) are evident. Embryonic endoderm is present for the first time.

The stage is represented in the database by Carnegie embryo #7801 that has a grade of excellent. The embryo is in the presomite, disc period. Because the gastrulation streak is present the specimen is classified as Stage 6b. The embryonic disc is oval, measuring 0.22 mm in width and 0.253 mm in length (Heuser et al., 1945). It is symmetrical and slightly convex dorsally. The thickness of the disc is nearly uniform measuring approximately 0.04 mm (Table of embryo dimensions).

This specimen was prepared for microscopic examination in 1940. It was fixed initially in 70% alcohol followed by Bouin's fluid. It was then embedded in celloidin/paraffin and serially sectioned transverse to the longitudinal axis at 8 microns. More than 300 sections were cut through the implantation site with 31 sections through the embryonic disc. The sections were mounted on glass slides and stained with hematoxylin and eosin. In this database structures are identified on 50 sections through the embryonic disc and adjacent tissue. These sections are located on slides 11, 12 and 13.

The morphology of this embryo is well documented in the literature. There are photographs of the implantation site and graphic reconstructions of the specimen. A drawing through the median plane showing the right half of the embryo was made from a model by Mr. J.F. Didusch in 1944. We have generated a digital reconstruction of the median longitudinal section corresponding to this drawing.

The sections have been digitally restored and labeled, and can be viewed at three magnifications. [Note: A fourth series of images show the embryo and surrounding tissues at the same magnification as zoom level 3.] Several 3D reconstructions have been produced from the aligned sections. Animations of these 3D-reconstructions together with fly-through animations of the aligned sections are also included on the disks.

Source: The Virtual Human Embryo.