XTTI 



PHYLUM CHORDATA 



73 



In connection with the anterior end of the notochord, where no 

 vertebra are formed, there are developed certain elements of the 

 skull or cephalic skeleton, a structure which is eminently charac- 

 teristic of the whole craniate division, and to the possession of 

 which it owes its name. The skull makes its first appearance in the 

 embryo in the form of paired cartilaginous plates, the parachordals 

 (Fig. 772, pc), lying one on each side of the anterior end of the 

 notochord (nch), and thus continuing forward the line of vertebral 

 centra. In front of the parachordals are developed a pair of curved 

 cartilaginous rods, the trabeculce (tr), which underlie the anterior 

 part of the brain, as the para- 

 chordals underlie its posterior 

 part : their hinder ends diverge 

 so as to embrace the pituitary 

 body (pty) already referred to. 

 Cartilaginous investments are 

 also formed around the organs 

 of the three higher senses : a 

 pair of olfactory capsules round 

 the organs of smell, one of 

 optic capsules round the organs 

 of sight, and one of auditory 

 capsules (au. c.) round the 

 organs of hearing. The optic 

 capsule, which may be either 

 cartilaginous or fibrous, re- 

 mains free from the remaining 

 elements of the skull in accord- 

 ance with the mobility of the 

 eye ; it constitutes, in fact, the 

 sclerotic or outer coat of that 

 organ. The olfactory capsules 

 are usually formed in relation 

 to the trabeculae, and are con- 

 tinuous with those structures 

 from an early stage. The 

 auditory capsules in some cases arise as outgrowths of the para- 

 chordals, in others as independent cartilages, each of which, how- 

 ever, soon unites with the parachordal of its own side. As 

 development goes on, the trabeculae and parachordals become fused 

 into a single basal plate (Fig. 773, B, b. cr.) underlying the brain : 

 the skull-floor thus formed gives off vertical up-growths on each 

 side which finally close in above to a greater or less extent, and so 

 give rise to a more or less complete cranium or brain-case enclosing 

 the brain and the organs of smell and hearing, and furnishing open 

 cavities or orbits for the eyes. 



In the continuous solid cranial box thus formed certain definite 



rich, 



[Q. 772. The elements of the cranium in an 

 embryo Salmon, from above, au. c. auditory 

 capsule ; nch. notochord ; pc. parachordal ; pty. 

 position of pituitary body ; tr. trabecula. (From 

 a model by Ziegler.) 



