The human chondrocranium at the end of the embryonic period, proper, with particular reference to the nervous system

The chondrocranium at stage 23 (27-32 mm C.-R., 8 postovulatory weeks) was studied in detail to fill the descriptive gap between 7 and 9 weeks. Serial sections of ten embryos, all belonging to the same developmental stage, were investigated and compared. Graphic and Perspektomat reconstructions were made from three embryos. The chondrocranium forms a continuous mass of well-formed cartilage, which blends with early cartilage in the nasal, orbital, and otic regions, around the future foramen rotundum, and at the site of the parietal lamina. The chondrocranium and "membrane" bones cover a relatively small portion of the brain, namely 1) the medulla and pons, 2) the caudal half of the lateral portion of the cerebellar primordium, and 3) the rostroventral part of the cerebral hemispheres. The relationships of the cranial nerves were are also investigated. The degree of cranial development is intermediate between that at stage 20 (21 mm) studied by Lewis (the only other skull so far studied adequately in a staged embryo) and that of fetuses of 40 and 43 mm investigated by Macklin. On the whole, the resemblances to the fetal skulls are greater: ossification is beginning in the tectum posterius (to form the supraoccipital), and most of the "membrane" bones (except the parietal, tympanic, lacrimal, and nasal) show osseous areas. The foramen rotundum is present. The lower angle between the chordal and prechordal parts of the basal stem ranges from 129 degrees (which resembles that at stage 20, namely 122 degrees) to 115 degrees (which is the same as that at 40-43 mm). An orbito-parietal commissure is not yet present. The lower border of the larynx is at the level of the lower border of the fourth cervical vertebra. In the larynx, the thyroid laminae have usually not fused, the hyoid cartilage possesses greater and lesser cornua, the cricoid cartilage displays its signet-ring form, and the arytenoids are identifiable as condensed mesenchyme with some early cartilage. The cartilage of the first pharyngeal arches is covered in its ventral three-fourths by an osseous mandibular plate. Morphological variations within stage 23 are minor and appeared not to be related closely to each other, so that it was not possible to arrange the embryos in a linear developmental series based on their level of cranial morphogenesis.