Carnegie Stage 23 Introduction
Stage 23 embryos have a greatest length of 23 to 32.2 mm and an estimated postfertilization age of 53 to 58 days. Over 90% of the more than 4,500 named structures in the adult body are present. The head is larger, more rounded and the subcutaneous venous plexus lies close to the vertex. The limbs are longer and the hands and feet can overlap each other. The external genitalia is prominent but not yet sexually distinct. The insula and subdivisions of the lateral ventricle can be identified in the cerebral vesicle (hemisphere). The superior and inferior colliculi are evident in the tectum of the mesencephalon and the external granular layer of the cerebellum has formed in the alar plate of the metencephalon. Many nuclei, fiber tracts, decussations and commissures are evident in the brainstem. Most of the named arteries to the brain can be identified. The upper and lower eyelids may exhibit laterally and medially the beginning of fusion with each other. All four of the parasympathetic ganglia in the head region are present. The cochlear duct attains its definitive arrangement of 2 ½ spirals. The palatine shelves are fusing with each other and the nasal septum. The salivary gland ducts have begun secondary branching. All named muscles can be identified. The laryngeal cartilages are present and ossification has begun in several places in the skull and long bones. The left superior vena cava is no longer present. The kidney exhibits 4 to 5 orders of tubules. Tubules can be identified in the testis but are not yet fused with the rete testis and mesonephric duct.
The stage is represented by Carnegie embryo #9226 that has a grade of excellent. It has a greatest length of 31 mm after fixation and is considered to be advanced in the stage. The estimated postfertilization age is 56 days. The embryo was prepared for microscopic examination in 1954. It was fixed in formol, embedded in celloidin and paraffin and serially sectioned transverse to the long axis at 12 microns. The sections were mounted on 209 large glass slides and stained with Azan. There are 2090 sections through the embryo. The DREM database includes 209 of the section images. Approximately every 10th section was digitally restored and labeled, and can be viewed at four magnifications in the Browse part of the database. The aligned section images were used to produce 3D reconstructions. Animations of the reconstructions and flythrough animations of the aligned section images are also included on the disks.
Various aspects of the morphology of the embryo were studied extensively. Selections of figures from most of the publications are included on the disk.
Source: The Virtual Human Embryo.