18!) 



LECTURE X. 



ON THE STRUCTURE OF THE SKULL. 



THE SKULLS OF FISHES. 



It Las been seen that the skull of Man and that of the Pike 

 agree in passing, in a similar order, through similar develop- 

 mental stages. Each, at first, is a membranous cranium, its 

 walls being composed of indifferent tissue, with the exception 

 of that small part of its base which is occupied by the notochord. 

 The greater part of the substance of each becomes chondrified, 

 and thus that cartilaginous cranium is produced, which is a 

 temporary structure in the Man, but a persistent one in the 

 Fish. Neither in the membranous, nor in the cartilaginous 

 state, does the cranium of either Man or Fish present any trace 

 of that segmentation which becomes obvious in the third con- 

 dition, when, by the deposit of calcareous salts around certain 

 centres, either in the cartilaginous cranium or the adjacent 

 membrane, the bony cranium is developed. 



These three conditions of the skull are manifested, in the 

 same order of succession, by all vertebrate skulls which become 

 completely ossified ; but the crania of many vertebrated animals 

 remain throughout life in the second state, or in a condition 

 intermediate between that and the third, while the skull of one 

 of the Vertebrata persists in a state which can only be regarded 

 as a modification of the membranous cranium. Hence I shall 

 proceed to describe the leading modifications of the Vertebrate 

 Skull under these heads: — A. The membranous cranium. B. 

 The cartilaginous cranium. C. The cartilaginous cranium, with 



