184 ON THE STRUCTURE OF THE SKULL. 



special structural peculiarities are superadded in each case. 

 Both types of skull exhibit many bones in common, but in each 

 type, some of these bones acquire special arrangements and very 

 different relative magnitudes ; and each type exhibits bones 

 peculiar to itself, the number of those present in the Fish and 

 absent in the Man, being very much greater than of those pre- 

 sent in the Man and absent in the Fish. As might be expected, 

 the study of the development of the Fish's cranium brings out 

 into still stronger light the fundamental resemblances of its 

 structure with that of the higher Vertebrates. The primitive 

 groove makes its appearance on the blastoderm, and becomes 

 converted into a canal by the arching over and coalescence of 

 the dorsal laminae. The anterior part of the canal dilates and 

 becomes subdivided into cerebral vesicles. The notochord ap- 

 pears and terminates, in front, in a point behind the pituitary 

 body ; while round its apex, that bend of the primitive cranium 

 takes place which constitutes the cephalic flexure. The organs 

 of sense make their appearance in the same regions, and the 

 visceral arches and clefts are developed in the same way. But 

 a greater number of them appear, and the posterior ones, 

 instead of vanishing, give rise to the branchial skeleton and 

 branchial clefts. The mandible is developed in the first visceral 

 arch, and the hyoid apparatus in the second, as in Man ; but 

 the details of the mode of origin of the hvomandibular and sym- 

 plectic, of the palatine and maxillary apparatus, and of the 

 naso-frontal process, have not been as yet worked out with 

 sufficient thoroughness to enable us to determine with certainty 

 the homologies of all the resulting parts. 



The cranium is at first wholly membranous, but after a time 

 it becomes partially chondrified in the same way as in the 

 higher Vertebrates (Fig. 72). Cartilage appears in the base of 

 the skull upon each side of the notochord, and surrounds the 

 great auditory capsules. Anteriorly it divides into two processes, 

 the trabecular cranii (Tr.), which separate so as to inclose the 

 pituitary fossa (P), and reunite, in front of it, to form the ethmo- 

 vomerine rostrum. From the floor of the skull, at the front and 

 lateral part of each auditory capsule, a cartilaginous process 

 (1IM.) is given off, and passing downwards and forwards ends in 



