PROGRESS OF MICROSCOPICAL SCIENCE. 143 



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

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

 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 cerato-hyal. 



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



(^) The olfactory cajisules have the turbinal outgrowths all marked 

 out as alinasal, nasal, and upper, middle, and lower turbiuals. 



4. In pigs of larger size the form and proj)ortions 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 develoi^ment, 

 consisting, as it does, of 



(a) A cartilaginous basicranial plate embracing the notochord, 

 and, like it, stopj)ing behind the pituitary body. 



(&) Paired cartilaginous arches, of which two are j)r8eoral, while 

 the rest are jDOstoral. 



(c) A pair of cartilaginous auditory capsules. 



(d) A pair of cartilaginous nasal capsules. 



Further, that in the Mammal, as in the other Vertebrata, the 

 development of the skull of which has been examined, the basicranial 

 plate gi'ows up as an arch over the occipital region of the skull, and 

 coalesces with the auditory capsules, laterally, to give rise to the 

 primordial skeleton of the occipital, periotic, and basisphenoidal 

 regions of the skull. The trabeculse become fused together, and, 

 imiting with the olfactory capsules, give rise to the presi)henoidal 

 and ethmoidal parts of the cranium ; and the moieties of the skull, 

 thus resulting from the metamorphosis of totally diiferent morpho- 

 logical elements, become united, and give rise to the primordial 

 cranium. 



As in the Salmon and Fowl, the second pair of prteoral arches 

 give rise to the ptery go-palatine apparatus. In the Frog this arch is 

 late in appearance, and is never distinct from the trabecular and man- 

 dibular bars, serving as a conjugational band between them. The 

 mandibular arch, which in the Salmon becomes converted into 

 Meckel's cartilage, the os articulare, the os quadratum, and the os 

 metapterygoideum, in the Frog into Meckel's cartilage and the qua- 

 drate cartilage (which early becomes confluent with the periotic cap- 

 sule), in the Bird into Meckel's cartilage, the os articulare, and the 

 OS quadratum (which articulates movably with the periotic capsule), 

 in the Pig is metamorphosed into Meckel's cartilage and the malleus, 

 which is loosely connected with the tegmen tympani, an outgrowth of 

 the periotic capsule. 



Meckel's cartilage persists in the Fish and in the Amphibia, but 

 disappears early in the Bird, and still earlier in the Mammal. The 



