431 



column we are to find a parallel for the double parietals and frontals. 

 But waiving this difficulty, and supposing, for the sake of argument, 

 as was supposed by Oken, that the basisphenoid, alisphenoid and 

 parietals, the presphenoid, orbitosphenoids, and frontals represent 

 the elements of two vertebral centra and neural arches, what is to 

 be made of the petrous and mastoid bones? 



The difficulty has been eluded by terming the petrosal a " sense- 

 capsule," the mastoid a "parapophysis." But I apprehend that 

 neither of these explanations can be received for a moment by those 

 who are acquainted with the development of the skull, or with the 

 true homologues of the bones in question in the vertebrate series, 

 or who think that scientific terms should always possess a well-de- 

 fined and single meaning. 



What, in fact, is the origin of the petrous and mastoid bones ? 

 There is much reason for believing (according to Remak's late obser- 

 vations) that the membranous labyrinth is primarily an involution 

 of the sensory or epidermic layer of the blastoderm ; but however 

 this may be, it is quite certain that the auditory organ is, primarily, 

 altogether independent of the walls of the skull, and that it may be 

 detached without causing any lesion of them, in young embryos. 



It is also quite certain that this membranous labyrinth becomes 

 invested by a coat of cartilage, continuous with the cranial wall ; but 

 I do not know that there is evidence, at present, to enable one to say 

 positively, whether this cartilaginous auditory capsule is formed in- 

 dependently around the labyrinth, and then unites with the cranium ; 

 or whether it is an outgrowth from the cranial walls, which invests 

 and encloses the labyrinth. If the latter be the case, a consistent 

 vertebral theory of the skull must account for all the bones deve- 

 loped out of the auditory capsule ; if the former, it must exclude 

 them all, as parts of an extra- vertebral sensory skeleton. 



Now the bones developed in the capsule are, in front, the petrosal ; 

 behind, the mastoid ; above, the epiotic. The first-named bone is 

 admitted, by the most zealous advocates of the vertebral theory, to 

 be a neurapophysis, in all oviparous Vertebrata. Hence they are 

 also bound to admit that, for three centra below and three neural 

 spines bounding the cranial cavity above, there are four pairs of 

 neural arches. More than this, I do not see how it is to be denied 

 that the true mastoid is the morphological equivalent of the petrosal ; 



