252 Royal Society : — 



((/) The proximal end of the hyoideau arch, similarly enlarging 

 and articulating wdth the corresponding part of the mandibular 

 arch, becomes the incus, the incurved process 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 

 hyoideau arch, becomes separated from the rest of the arch by con- 

 version of part of the arch into fibrous tissue, and by the moving 

 dowTiwards and backwards of the proper hyoid portion of the arch. 

 A nodule of cartilage left in the fibrous connecting band becomes a 

 styliform intevhyal cartilage, while the proximal end of the detached 

 arch becomes the stiflo-ht/al. 



(h) The tliyro-hyals have merely increased in size and density ; 

 they closely embrace the larynx by their u]iper ends. 



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

 inner edges ha^-e coalesced with each other and, below, with the 

 trabeculae to form the great median septum : the turbinal out- 

 growths are apparent. 



In this stage, the aKsphenoids and orbito-sphenoids appear as 

 chondrifications of the walls of the skvdl, quite separate from the 

 investing mass and from the trabeculae. 



The floor of the pituitary space choudrifies independently of the 

 trabeculse and investing mass, but serves to unite these foiu* cartila- 

 ginous tracts. 



3. In an embryo pig, 1\ inch in length, («, h, c) the primordial 

 craniiun is completely constituted as a cartilaginous whole, formed 

 by the coalescence of the investing mass and its exoccipital and 

 superoccipital prolongations, the modified trabeculfe, the subpitui- 

 tary cartilage, the auditory capsules, and alisphenoidal and orbito- 

 spheuoidal cartilages, and the olfactory capsules. The uotochord is 

 yet to be seen extending in the middle line from the hinder wall of 

 the pitiutary fossa (now the " dorsum selke ") to the posterior edge 

 of the occipital region. 



{d) The trabecular arches form the sides of the sella turcica, 

 the praesphenoid, and the base of the septum between the olfactory 

 capsides ; in front, where they form the azygous " prsenasal," th<fy 

 are de\eloped backwards as " recurrent bands," elongations of their 

 fi'ee recurved " cornua." 



{e) The pterygo-palatine arches, still increasing in size, but not 

 chondrifying, are rapidly ossifying; they are half-coiled laminje 

 bounding the posterior nasal passages. 



(/) The mandibular arch and the rudimental ramus have become 

 solid cartilage, and the latter is ossif}dng as the dentary ; the 

 distal part of each mandibular rod unites ^rith its fellow for some 

 distance. 



(g) The hyoid arches are each fully segmented as incus, with 

 its " orbicular " head, interhyal, stylo-hyal, and cerato-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 tur- 

 binals. 



