298 ON THE STRUCTURE OF THE VERTEBRATE SKULL. 



on the special characters of a skull, and the other those of a ver- 

 tebral column ; the latter taking one road, while the skull takes 

 another. The skull is no more a modified vertebral column 

 than the vertebral column is a modified skull ; but the two are 

 essentially separate and distinct modifications of one and the 

 same structure, the primitive groove. 



6. The skull, having assumed its special and distinctive 

 characters, may pass through three successive states — the mem- 

 branous, the cartilaginous, and the osseous — in the course of 

 its development ; and the order in which these states succeed 

 one another is always the same, so that the osseous skull has 

 a cartilaginous, and the cartilaginous, a membranous, pre- 

 decessor. Nor does any one of these states ever completely 

 obliterate its predecessor; more or less cartilage and mem- 

 brane entering into the composition of the most completely 

 ossified skull, and more or less membrane being discoverable in 

 the most completely chondrified skull. 



7. The adult skull may, however, have got no further than 

 one of these states. In the Ampliioxus, the skull (if skull it 

 can be called) is membranous. In many Fishes, as we have 

 seen, it is cartilaginous, with, at most, a superficial conversion 

 into bone. In the rest of the Vertebrata definite bones are 

 added, to the more or less complete exclusion of the cartilaginous 

 cranium. 



8. When definite cranial bones are developed, they arise in 

 one of two ways, either in the substance of the cartilaginous 

 cranium, as "cartilage bones," or in the perichondrium, or 

 remains of the membranous cranium, as " membrane bones." 

 It is highly probable that, throughout the vertebrate series, 

 certain bones are always, in origin, cartilage bones, while certain 

 others are alwavs. in origin, membrane bones. 



9. With the exception of Amjihioxus, three sets of sensory 

 organs — olfactory, optic, and auditory — are evolved in the 

 walls of the skull of every vertebrate animal, and they are 

 disposed, from before backwards, in the order in which they are 

 named. All these sensory organs are originally developed in 

 connection with involutions of the integument, which, in the 

 case of the olfactory organ, remain open, but, in that of the eye 



