1873.] 



of the Skull in the Pig . 



405 



(i) The olfactory capsules are well chondrified ; their descending inner 

 edges have coalesced with each other and, below, with the trabecule to 

 form the great median septum : the turbinal outgrowths are apparent. 



In this stage, the alisphenoids and orbito-sphenoids appear as chon- 

 drifications of the walls of the skull, quite separate from the investing 

 mass and from the trabecule. 



The floor of the pituitary space chondrifies independently of the 

 trabecule and investing mass, but serves to unite these four cartilaginous 

 tracts. 



3. In an embryo pig, 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 trabecule, the subpituitary cartilage, the 

 auditory capsules, and alisphenoidal and orbito-sphenoidal cartilages, and 

 the olfactory capsules. The notochord is yet to be seen extending in the 

 middle line from the hinder wall of the pituitary fossa (now the " dorsum 

 settee ") 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 " prsenasal," they are developed 

 backwards as "recurrent bands," elongations of their free recurved 

 " cornua." 



(e) The pteiygo-palatine arches, still increasing in size, but not 

 chondrifying, are rapidly ossifying ; they are half -coiled laminae bounding 

 the posterior nasal passages. 



(/) The mandibular arch 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 each fully segmented as incus, with its 

 " orbicular" head, interhyal, stylo-hyal, and<3erato-hyal. 



(7i) The thyro-hyals are merely larger and denser. 



(i) The olfactory capsules have the turbinal outgrowths all marked out 

 as alinasal, nasal, and upper, middle, and lower turbinals. 



4. In pigs of larger size the form and proportions of 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 condition, is strictly comparable with that of an Osseous Fish, a 

 Frog, or a Bird at a like period of development, consisting as it does of 



(a) A cartilaginous basicranial plate embracing the notochord, and, 

 like it, stopping behind the pituitary body. 



(b) Paired cartilaginous arches, of which two are praeoral, while the 

 rest are postoral. 



(c) A pair of cartilaginous auditory capsules. 



(d) A pair of cartilaginous nasal capusles. 



VOL. XXI. 2 K 



