DEVELOPMENT OF THE ENDOSKELETON 671 



dition in which dense mesenchyme, the so-called desmocranium, comes to 

 surround the brain and its appendages. The membranous cranium is more 

 pronounced in the basal areas of the brain. This pre-cartilage stage is followed 

 by formation of cartilage which results in the development of a chondric neuro- 

 cranium. A complete cartilaginous neurocranium is not formed in all verte- 

 brate groups, although the ventro-lateral areas of all vertebrate skulls are laid 

 down in cartilage. This basic, chondrocranial condition exists as the first step 

 in skull formation, and it consists of three main regions, composed of car- 

 tilaginous rudiments (figs. 316A, 320): 



( 1 ) The basal plate area is composed of a pair of parachordal cartilages 

 on either side of the anterior extremity of the notochord, together 

 with the otic capsules, surrounding the otic (ear) vesicles. 



(2) A trabecular or pre-chordal plate area lies anterior to the notochord. 

 This area begins at the infundibular-hypophyseal fenestra and extends 

 forward below the primitive forebrain. Two elongated cartilages, the 

 trabecula cranii (fig. 320A) or a single elongated cartilage (fig. 320B), 

 the central stem or trabecular plate, develop in the basal area of this 

 region. With the trabecular area are associated the sphenolateral, 

 orbital or orbitosphenoidal cartilages and the optic capsules. The 

 latter are placed in a position lateral to the orbitosphenoidal cartilages. 



(3) A nasal capsular or ethmoidal plate area, associated with the develop- 

 ing olfactory vesicles, later arises in the anterior portion of the tra- 

 becular region (figs. 316A, 319A). 



This fundamental cartilaginous condition of the vertebrate skull or 

 neurocranium is followed by later conditions which proceed in three 

 ways: (a) In the elasmobranch fishes, an almost complete roof of 

 cartilage is developed, and the various cartilaginous elements fuse to 

 form the cartilaginous neurocranium (fig. 315). This neurocranium 

 enlarges but never becomes ossified, (b) In the ganoid fish, Amia, 

 the frog, Rana, the mud puppy, Necturus, etc., the basic, ventro- 

 laterally established, cartilaginous neurocranium is converted into a 

 more or less complete chondrocranium by the formation of a roof 

 and the complete fusion of the various cartilaginous elements (figs. 

 316A-C; 317A, B). In these forms, the cartilaginous cranium be- 

 comes ossified in certain restricted areas. In addition to this cartilagi- 

 nous neurocranium, superficial, membrane bones (dermal bones) are 

 added to the partially ossified chondrocranium. These membrane bones 

 come to overlie and unite with the partly ossified cartilaginous skull 

 (figs. 316D; 317C). (Consult also Table 1.) The adult skull or neuro- 

 cranium in these forms thus is composed of a chondrocranial portion 

 and an osteocranial part, the osteocranial part arising from cartilagi- 

 nous and membranous sources, (c) In reptiles, birds, mammals, and 



