DEVELOPMENT OF THE SKULL IN THE PIG. 
329 
attaching itself to the outer surface of the stapes and becoming the long process of the 
incus. The incus, thus formed out of the proximal end of the hyoidean arch, becomes 
separated from the rest of the arch by conversion of part of the arch into fibrous tissue, 
and the moving downwards and backwards of the proper hyoid portion of the arch. A 
nodule of cartilage left in the fibrous connecting band becomes a styliform “interhyal ” 
cartilage, while the proximal end of the detached arch becomes the stylohyal. 
(h) The thyrohyals have merely increased in size and density ; they closely embrace 
the larynx by their upper ends. 
(?') The olfactory capsules are well chondrified, and their descending inner edges have 
coalesced with each other and with the trabeculae below to form the great median 
septum : the turbinal outgrowths are apparent. 
In this stage the alisphenoids and orbito-sphenoids appear as chondrifications of the 
Avails of the skull, quite independent of the investing mass and the trabeculae. 
The floor of the pituitary space chondrifies independently of the trabeculae and the 
moieties of the investing mass, but serves to unite these four cartilaginous tracts. 
3. In an embryo pig 1^ inch in length, (a, b , c) the primordial cranium is completely 
constituted as a cartilaginous whole, formed by the coalescence of the investing mass 
and its exoccipital and superoccipital prolongations, the modified trabeculae, the sub- 
pituitary cartilage, the auditory capsules, the alisphenoidal and orbito-sphenoidal carti- 
lages, and the olfactory capsules. The notochord is still to be seen extending in the 
middle line from the hinder wall of the pituitary fossa (now the clinoids of the sella 
turcica) to the posterior edge of the occipital region. 
(d) The trabecular arches form the sides of the sella turcica, the presphenoid, and 
the base of the septum betAveen the olfactory capsules ; in front, where they form the 
azygous prenasal or “ basitrabecular ” element, they are developed backAvards as 
“ recurrent bands,” elongations of the free recurved cornua. 
( e ) The pterygo-palatine arches, still increasing in size, but not chondrifying, iioav 
rapidly ossify; they are half-coiled laminae bounding the posterior nasal passages. 
(f) The mandibular arches and the rudimental ramus have become solid cartilage, 
and the latter is ossifying as the dentary ; the distal part of each mandibular rod unites 
with its fellow for some distance. 
( g ) The hyoid arches are now more fully segmented as incus, with its orbicular 
head, interhyal, stylohyal, and ceratohyal. 
(h) The thyrohyals are merely larger and denser. 
(i) The olfactory capsules have now the turbinal outgroAvths all marked out as ali- 
nasal, nasal, upper, middle, and lower turbinals. 
4. In pigs of larger size the form and proportions of all the parts of the cranium 
become greatly altered, and ossification takes place on an extensive scale, but no neAv 
structure is added. 
o. It follows from these facts that the mammalian skull, in an early embryonic con- 
dition, is strictly conformable with that of an Osseous Fish, a Frog, or a Bird at a like 
period of development, consisting as it does of : — 
MDCCCLXXIV. 2 x 
