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 trabeculge 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 

 walls of the skull, quite independent of the investing mass and the trabeculee. 



The floor of the pituitary space chondrifies independently of the trabeculse 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 trabeculse, 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 between the olfactory capsules; in front, where they form the 

 azygous prenasal or " basi trabecular " element, they are developed backwards as 

 " recurrent bands," elongations of the free recurved cornua. 



(e) The pterygo-palatine arches, still increasing in size, but not chondrifying, now 

 rapidly ossify ; they are half-coiled laminse 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. 



(?) The olfactory capsules have now the turbinal outgrowths 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 new 

 structure is added. 



5. 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 : — 



JIDCCCLXXIV. 2 X 



