CH. LV1II.1 



FORMATION OF HEAD. 



827 



and two other bars embracing the pituitary body situated in front 

 of these are called the trabeculce cranii (fig. 643, A). These unite 

 in front, and with the parachordal cartilages behind to form a 

 continuous mass (basilar cartilage), which completely invests the 

 notochord posteriorly (fig. 643, B). The parachordal part of this 

 represents the basi-occipital and basi-sphenoid ; the prechoi-dal 

 part represents the pre-spheuoid and ethmoid portions ; posteriorly 

 and at the sides, cartilaginous plates grow over the cerebral 

 vesicles, but this only occurs to a small extent in mammals. In 

 these animals the occipital region alone is roofed in by cartilage ; 

 the rest of the cranial vault being formed of membrane bones. 



Anteriorly the united trabeculse form the ethmoid cartilage and 

 the nasal septum, and enclose the nasal pits externally. 



From the sides of the pre-sphenoid, the lesser wings or orbito- 



A. 



Fig. 643. Diagrams of the cartilaginous cranium. 



A, first stage. Ch, notochord ; Tr, trabeculee cranii ; P.ch., parachordal cartilages; P, 



situation of pituitary body ; N, E, O, situations of olfactory, visual, and auditory 

 organs. 



B, later stage. B, basilar cartilages ; S, nasal septum and ethmoidal cartilages ; Kth, 



Elk', prolongations of ethmoid around olfactory organ, completing the nasal capsule ; 

 N, E, O, Ch, Tr, P, as before. (After Wiederaheim.) 



sphenoids containing the optic foramina are developed, and from 

 the sides of the basi-sphenoid the greater wings or alisphenoids. 

 A cartilaginous capsule invests the auditory vesicle and becomes 

 connected to the parachordal cartilage on each side. It is called 

 the periotic capsule ; within this bony centres are formed, and 

 the bone constitutes the petrous and mastoid portions of the 

 temporal bone. 



The Visceral Clefts and Arches. In all vertebrata there 

 is at one period of development a series of slits in the neck region; 

 these are formed as inpushings from the exterior, and open into 

 the anterior end of the alimentary canal. These are six in 

 number, but in man the two hinder ones rapidly disappear ; the 

 first enlarges and forms the mouth ; and at the sides of this the 



