Carnegie Stage 13 Introduction
Stage 13 embryos have a greatest length of 3.9 to 6 mm and an estimated postfertilization age of 30 to 33 days. There are 30 or more pairs of somites or body segments along the body axis. The retinal part of the optic vesicle and the lens disc can be identified. The caudal neuropore is closed resulting in a completely closed neural tube. The otic vesicle has separated from the overlying surface ectoderm and the otic capsule is forming around the vesicle as a mesenchymal condensation. The tuberculum impar (median tongue eminence) and copula are present in the floor of the pharynx. The lower limb bud has formed. The primary interatrial foramen and septum form as the heart enlarges. The fourth aortic arch is prominent in the fourth pharyngeal arch. The midgut mesentery contains an arterial plexus derived from the aorta and is the precursor of the arteries to the gastrointestinal tract. The stomach and omphalomesenteric duct are present. The right and left lung buds form, which are the precursors of the primary bronchi. The mesonephoros contains glomeruli and S-shaped tubules. The urorectal septum has begun to separate the cloaca into urogenital and rectal portions.
The stage is represented by Carnegie embryo #836 that has a grade of excellent. It was selected for digital replication because it is one of the best specimens at this stage in the collection. It is considered to be representative of the early part of the stage. There are 32 pairs of somites or dermatomyotomes along the body axis. The embryo was prepared for microscopic examination in 1914. It was fixed in a saturated aqueous solution of mercuric chloride containing 5 per cent glacial acetic acid and hardened in alcohol that was gradually increased from 50 to 70 per cent. It was embedded in paraffin and serially sectioned transverse to the long axis at 15 microns. The 247 sections were mounted on 14 large, glass slides and stained with alum cochineal (i.e., carmine).
The morphology of the embryo is well documented in the literature. Multiple photomicrographs taken before and after sectioning are available and details of several reconstructions of the specimen have been published previously.
Source: The Virtual Human Embryo.