. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. OSSIFICATION OF THE CHONDROCRANIUM 175. first formed, the three cartilages are quite separate from one another, the olfactory and nasal nerves passing down between them to the olfactory pit, but later trabecular begin to extend across from the mesethmoid to the upper part of the ectethmoids and eventually form a fenestrated horizontal lamella which ossifies to form the cribriform plate. The lower part of the median cartilage does not ossify, but a center appears on each side of the median lin
. The development of the human body : a manual of human embryology. Embryology; Embryo, Non-Mammalian. OSSIFICATION OF THE CHONDROCRANIUM 175. first formed, the three cartilages are quite separate from one another, the olfactory and nasal nerves passing down between them to the olfactory pit, but later trabecular begin to extend across from the mesethmoid to the upper part of the ectethmoids and eventually form a fenestrated horizontal lamella which ossifies to form the cribriform plate. The lower part of the median cartilage does not ossify, but a center appears on each side of the median line in the mesenchyme behind and below its posterior or lower border. From these centers two verti- cal bony plates develop which unite by their median surfaces below, and above invest the lower border of the cartilage and form the vomer. The portion of the cartilage which is thus invested undergoes resorption, but the more anterior portions persist to form the cartilaginous septum of the nose. The vomer, consequently, is not really a portion of the chondrocranium, but is a membrane bone; its intimate relations with the median ethmoidal cartilage, however, make it convenient to consider it in this place. When first formed, the ectethmoids are masses of spongy bone and show no indication of the honeycombed appearance which they present in the adult skull. This condition is produced by the absorption of the bone of each mass by evaginations into it of the mucous membrane lining the nasal cavity. This same process also brings about the formation of the curved plates of bone which project from the inner surfaces of the lateral masses and are known as the superior and middle conchse (turbinated bones). The inferior and sphenoidal conchae are developed from special centers, but belong to the same category as the others, being formed from por- tions of the lateral ethmoidal cartilages which become almost Fig. 108.—Anterior Portion of the Base of the Skull of a 6 to 7 Months' Embryo. T
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