142 PKOGEESS OF MICROSCOPICAL SCIENCE. 



and a pliig on tlie outer cartilaginous wall of each has become marked 

 off as the stapes. 



(d) The hinder ends of the trabecular arches have coalesced in 

 front of the pituitary body, but they are not yet confluent with the 

 investing mass. 



(e) The pterygo-palatine rods have increased in size ; they have 

 not become hyaline cartilage, but are beginning to ossify in their centre. 



(/) In the mandibular arch the proximal end has become some- 

 what bulbous, and is recognizable as the head of the malleus, whilst 

 the incui'ved process, still more prominent than before, is the manu- 

 brium mallei. The rest of the arch is Meckel's cartilage ; outside this 

 a mass of tissue appears, which is converted into cartilage, rapidly 

 ossifies, and eventually becomes the ramus of the mandible. 



(g) The proximal end of the hyoidean arch, similarly enlarging 

 and articulating with 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 hyoidean arch, 

 becomes separated from the rest of the arch by conversion of part of 

 the arch into fibrous tissue, and by the moving downwards and back- 

 wards 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 

 stylo-hjal. 



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

 they closely embrace the larynx by their upper ends. 



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

 inner edges have coalesced with each other, and below with the trabe- 

 cule 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 separate from the 

 investing mass, and from the trabecular. 



The floor of the pituitary simce chondrifies independently of the 

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

 ginous 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 exoccijjital and super- 

 occipital prolongations, the modified trabecula3, the subjiituitary carti- 

 lage, 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 pitui- 

 tary fossa (now the " dorsum sellce. ") to the posterior edge of the 

 occipital region. 



((/) The trabecular arches form the sides of the sella turcica, the 

 jjresphenoid, and the base of the septum between the olfactory cap- 

 sules ; in front, where they form the azygous " prienasai," they are 

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

 recurved " cornua." 



