216 THE MESODERMAL DERIVATIVES 



Other pair of rods of cartilage and then bones. These are the trabeculae 

 (or trabecular cartilages) which join mesially to form the ethmoid or 

 intranasal plate. The anterior space between the trabeculae is the 

 basicranial fontanelle within whose cavity the infundibulum tempo- 

 rarily lodges. 



The sense capsules are simultaneously formed. Each auditory cap- 

 sule forms from mesenchyme which surrounds the developing inner 

 ear. It consists of mesotic and occipital cartilages. The occipital carti- 

 lages form the side and roof of the auditory capsule and skull and the 

 mesotic forms the floor. Between the occipital cartilages of the two 

 sides is the large posterior opening for the spinal cord to the brain, 

 known as the foramen magnum. Anteriorly the trabeculae form the 

 orbital bones around the eyes. The anterior extremity of these carti- 

 lages are the trabecular cornu which form the bony olfactory capsules. 

 Anteriorly they fuse with the labial or suprarostral cartilages. 



Some of the parts of the cranium are covered by thin plates which 

 originate in the dermis and not from the sclerotome and are there- 

 fore called dermal bones. Many of these cover open spaces of the 

 chondrocranium and line the mouth. They appear very early and 

 include the fronto-parietals, nasals, premaxillae, maxillae, quadrato- 

 jugals, squamosals, parasphenoids, vomers, dentaries, and palatines. 

 The main cartilage bones are the exoccipitals, prootics, stapes, eth- 

 moids, parts of the pterygoquadrates, articulars, hyoids, branchials, 

 and mento-Meckelians. 



The Visceral Arches. 



These arches give rise to the visceral skeleton (splanchnocranium) 

 but arise as six pairs of vertical condensations of mesenchyme lateral 

 to the embryonic pharynx. After the mouth opens each of these arches 

 develops cartilage. The most anterior pair, the mandibular arches, 

 give rise to the dorsal palatoquadrate bone and to the ventral Meckel's 

 cartilage. The palatoquadrate (pterygoquadrate) cartilage arises from 

 the maxillary (dorsal) portion of this first visceral arch and joins the 

 posterior end of the trabeculae. The palato portion gives rise to the 

 bulk of the upper jaw and the quadrate portion to the annulus tym- 

 panicus of the middle ear. The Meckel's cartilage gives rise to the core 

 of the lower jaw or the dentary bone. The large ceratohyal cartilages 

 appear posterior to the Meckelian cartilage in the hyomandibular 

 arch. The basicranial (copula) is an unpaired cartilage found between 



