CHONDROID TISSUE IN THE EARLY FACIAL MORPHOGENESIS OF THE CHICK-EMBRYO

The calcified tissues involved in the early morphogenesis of the so-ca lled intramembranous bones of the facial skeleton were studied by micr oradiographic and histological techniques in 22 chick embryos at the 9 th, 12th and 14 th days of incubation. On the 9th day, the bones of th e upper face and palatal vault are made up of thin sheets of chondroid tissue, deposited in their respective mesenchymal condensations. Wove n and lamellar bone formation subsequently takes place in each of them from the 12th day of incubation, mainly on the external side of their chondroid primordia. The same phenomena occur in the lower facial and mandibular bones. These facts indicate that the primitive facial desm ocranium of the chick embryo, which is classically considered to be fo rmed by intramembranous ossification, first consists of chondroid tiss ue. As in the cranial vault, this tissue thus represents the initial m odality of the skeletogenic differentiation within the avian facial me senchyme.