Stage 18 embryos have a greatest length of 13 to 17 mm and an estimated postfertilization age of approximately 44 days. Many structures become apparent for the first time during this stage. In the head region the tip of the nose becomes discernable as well as the volmeronasal organ, eyelids, and nasolacrimal ducts. The auricular hillocks begin to blend to form parts of the definitive external ear. Two or three semicircular ducts form from the otic vesicle in the otic capsule. The oronasal membrane breaks down leaving behind the choanae through which the nasal cavity will communicate with the pharynx. The subarachnoid space surrounds the entire central nervous system and the choroid plexus is forming in the roof of the 4th ventricle. The adenohypophyseal pouch is closed ending below as a stem. The mammary glands appears in the surface ectoderm on the side of the thorax. Metacarpals are evident as rays of precartilage condensations, or cartilage, in the hand plate and intermetacarpal notches are present at the periphery of the plate. Similarly, metatarsals appear as rays of precartilage condensations or cartilage in the foot plate with intermetatarsal notches at the periphery of the foot plate. The phallus and genital tubercle (glans penis and glans clitoris) are evident for the first time. Many individual muscles and cartilages that were earlier blastema can be identified. Internally, the second interatrial septum and foramen ovale can be identified in the heart and the left postcardinal vein near the heart has disappeared with its distal portion draining into the newly formed inferior vena cava. Tertiary segmental bronchi are distinct in the lung and the muscular part of the diaphragm is forming at the periphery of the septum transversum. The paramesonephric duct begins to form along the lateral side of the cephalic part of the mesonephros.
The stage is represented by Carnegie embryo # 4430 that has been given a grade of excellent. This female embryo is considered to be the one of the best specimens in the Carnegie collection at this stage. It has a greatest length of 14.0 mm (after fixation) and is in the most advanced one-third of the stage. In 1923 it was fixed in corrosive acetic acid, embedded in paraffin and serially sectioned transverse to the long axis at 15 microns. The sections were mounted on 29 glass slides and stained with alum cochineal (i.e., carmine). There are 837 sections through the embryo. Approximately every fifth or sixth section was digitally captured, restored and labeled. The Browse part of the database contains 147 section images that can be viewed at four magnifications. Several 3D Reconstructions were produced from aligned section images. Animations of the 3D reconstructions and flythroughs of the aligned section images are also included on the disk. For anyone who wishes to use them for other reconstructions, research, or presentations, the section images are available as individual .jpg files or as a self-extracting archive. An archive file of the 433 aligned, section images used for the 3D reconstructions is also available. A photograph of the right side of the embryo before it was sectioned and several drawings that were included in O'Rahilly and Müller Developmental Stages in Human Embryos, can be viewed in the Figures section of the disk. Instructions for using the disk and help with installation or other technical issues can be found on the Instructions pages.