The Development of the Osteocranium of the Egyptian Cobra
Authors: Kamal A. M. , Hammonda H. G. , Mokhtar F. M. 
Published: April 2010

The embryonic osteocranium of the elapid snake, Naja haje (Egyptian cobra) has been studied. The osteocranium of the 166‐mm embryo has been graphically reconstructed and described in detail. Those of younger and older embryos as well as the just‐hatched young have been also investigated. The membrane‐bones make their appearance before any sign of ossification in the cartilaginous cranium. The maxillary, palatine and pterygoid premordia have an earlier appearance than the other bony premordia.

The fused premaxillae arise beneath the antero‐medial part of the solum nasi of the nasal capsules. They send dorsally the rudiment of the processus nasalis and posteriorly two small processes. The nasal develops as a protecting bony roof for the cartilaginous nasal capsule. It possesses a median descending portion which penetrates deeply between the capsules. The frontal arises at first over the fenestra olfactoria advehens and the planum antorbitale, then extends posterioly. Its median descending portion has not yet ossified. The prefrontal embraces the posterolateral surface of the nasal capsule. It is perforated by the foramen of the lacrymo‐nasal duct, the lacrymal bone is completely absent. The parietal is very long forming the incomplete roof and the side wall of the temporal region. It is quite apart from the frontal. The cranial roof between the two frontals and two parietals is bridged by connective tissue. Both the frontal and parietal bones form well developed lateral down‐growths. The postfrontal is present, lying just lateral to the anterior part of the descending process of the parietal, but the postorbital is completely lacking.

There is only one temporal bone which is regarded as the supra‐temporal. This bone arises just lateral to the auditory capsule, its posterior part is intercalated between the quadrate and the capsule. The squamosal, tabular, jugal and quadratojugal are totally absent. The maxilla arises directly below the anterior half of the eye. It sends inwards a well developed palatal process. The rudiments of the maxillary teeth, including those of the poisonous fangs, have not yet been attached to the bone. The prevomer forms the medial wall and floor of the bony capsule which surrounds Jacobson's organ. It consists of a medial portion which extends nearly vertically and a ventral one which extends horizontally. The septomaxilla forms the lateral wall and a part of the floor of Jacobson's organ bony capsule. It possesses a very long posterior projection. The palatine arises medial to the maxilla, posteriorly it is about to touch the anterior tip of the pterygoid. The pterygoid is very long, occupying about half the total length of the palate. Both the palatine and pterygoid carry teeth which have not yet been completely attached to the bones. The ectopterygoid arises lateral to the hind extremity of the palatine and the anterior half of the pterygoid. The parasphenoid is a narrow elongated bone which arises in between the anterior halves of the trabeculae cranii. It is completely separate from the lateral down‐growths of the frontals and from the basisphenoid.

Other than the cartilage‐bone, the articular, each ramus of the lower jaw is composed of the following membrane‐bones: dentary, splenial, angular, and the combined supra‐angular and prearticular. The fusion between the supra‐angular and prearticular takes place in late embryos.

As regards the cartilage‐bones, the basioccipital ossifies perichondrally in the posterior part of the basal plate, behind the basicranial fenestra. The exoccipital ossifies in the occipital arch, and spreads medially forming a center in the posterior part of the tectum synoticum. The basisphenoid ossifies in the anterior region of the basal plate (including the crista sellaris) and the posterior ends of the trabeculae cranii. The basicranial fenestra is still present in its center. The supraoccipital arises from a pair of perichondral lamellae formed on the dorsal and ventral surfaces of the anterior part of the tectum synoticum. Laterally, on either side, it extends slightly in the roof of the auditory capsule, but this lateral part must not be considered as an epiotic since its independent ossification is not observed. The opisthotic arises from two perichondral lamellae on the outer and inner surfaces of the postero‐lateral part of the auditory capsule. It is quite separate from the exoccipital. The prootic ossifies perichondrally in the anterior portion of the auditory capsule, as far backwards as the anterior margin of the fenestra ovalis. The epipterygoid is completely lacking. The quadrate bone is formed by the perichondral ossification of the quadrate cartilage and the intercalare. The stapes and the shaft of the columella auris ossify perichondrally, but the processus dorsalis is still cartilaginous.

The laterosphenoid bone is wedged between the auditory capsule and the basisphenoid, it is completely fused with the prootic. It is of double origin; its ventral part arises by the perichondral ossification of the laterosphenoid cartilage while its dorsal part arises without cartilaginous preformation. The articular arises perichondrally round the hind part of Meckel's cartilage, it is distinct from the combined supra‐angular and prearticular. No ossification occurs in the hyoid apparatus.