62 Proceedings of the Royal 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 stud}^ 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 lamince 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 
hachvMrds throughout the whole length of the spinal column, 
and forwards to the tip of the cartilage of the nose." 
