INTRODUCTORY. DEVELOPMENT OF THE CRANIUM. 17 



of the brain it is called the membranous cranium. Ventral 

 to the central nervous system is the notochord, which extends 

 far into the region of the future cranium, and like the 

 nervous system, is enclosed by the skeletogenous layer. The 

 primitive cartilaginous cranium is formed by histological 

 differentiation within the substance of the membranous 

 cranium and always consists of the following parts : 



(a) the parachordals. These are a pair of flat curved 

 plates of cartilage, each of which has its inner edge grooved 

 where it comes in contact with the notochord. The para- 

 chordals, together with the notochord, form a continuous 

 plate, which is known as the basilar plate. The basilar 

 plate is the primitive floor below the hind- and mid-brain. 

 In front the parachordals abut upon another pair of carti- 

 laginous bars, the trabeculae, the two pairs of structures being 

 sometimes continuous with one another from the first ; 



(6) the trabeculae which meet behind and embrace the 

 front end of the notochord. Further forwards they at first 

 diverge from one another, and then converge again, enclosing 

 a space, the pituitary space. After a time they generally 

 fuse with one another in the middle line, and, with the para- 

 chordals behind, form an almost continuous basal plate. The 

 trabeculae generally appear before the parachordals. They 

 form the primitive floor below the fore-brain ; 



(c) the cartilaginous capsules of the three pairs of sense 

 organs. At a very early stage of development involutions 

 of the surface epiblast give rise to the three pairs of special 

 sense organs the olfactory or nasal organs in front, the optic 

 in the middle, and the auditory behind. The olfactory and 

 auditory organs always become enclosed in definite cartilagi- 

 nous capsules, the eyes often as in the Salmon, become 

 enclosed in cartilaginous sclerotic capsules, while sometimes, 

 as in mammals, their protecting capsules are fibrous. 



Each pair of sense capsules comes into relation with part 

 R. 2 



