62 Proceedings of the Boy al Physical Society. 



filled up by auditory or otic sense capsule, containing the 

 labyrinth or internal ear, with the tympanum and vaginal 

 process, in which the styloid is inserted ; this very complex 

 bone, though single in human anatomy, contains no less 

 than part of two vertebral neur-arcs, and the auditory appa- 

 ratus, in addition to the temporal limb zone ; at the same 

 time forming the barrier between the cerebral and cerebellar 

 cavities. 



Fully impressed as we are with the importance of the study 

 of development in determining the homologies of the skele- 

 ton, it may be doubted whether a sufficient acquaintance 

 with it has yet been acquired to enable us to trust to it as 

 the only sure guide. An erroneous theory of foetal develop- 

 ment cannot be expected to correct a mistaken homology, 

 however supported by names distinguished in science. 



Without presuming to maintain that the homology here 

 sketched will be received in all its details, it may be fear- 

 lessly maintained that no sound homology in fishes can be 

 sustained, without adopting some of the suggestions of the 

 foregoing communication, especially as regards the oper- 

 cular bones and pectoral fins in fishes — in regard to which 

 Cuvier, St Hilaire, Owen, Huxley, &c. &c, have completely 

 misapprehended the homology — in the study of a rather 

 complex subject, while, at the same time, it allows the fullest 

 scope for speculative interpretation of the laws of organic 

 development. 



It is generally admitted that the vertebral column is de- 

 veloped on the Chorda centralis, but that the first traces of 

 ossification in the embryo are the square bodies seen on each 

 side of the linear groove of the Blastoderm, " which even- 

 tually become the middle region of the head or mesocranium, 

 and the dorsal laminae produced, extending forwards or back- 

 wards like parapets on each side of the primitive groove, and 

 lay the foundations of the lateral walls not only of the skull 

 but of the spinal column. Very early the boundary between 

 the skull and spinal column is laid down. The proto-ver- 

 tebral elements increase in number from the mesocranium 

 backwards throughout the whole length of the spinal column, 

 and forwards to the tip of the cartilage of the nose." 



