NA TURE 



\Nov. 5, 1874 



appeared— the opisthotic (Op.O) and the minute epiotic 

 Ep.O) ; the latter attains a much greater size before it 

 fuses, in adult life, with the supra-occipital. The mam 

 part of the skull floor is formed by the large, laterally 

 expanded basi-sphenoid, which above is excavated into a 

 deep sella turcica for the pituitary body, and in front 

 passes into the interorbital septum and the bony rostrum 



Fig 25 —Skull of Fowl at the period of hatching(side view), p.p pars plana 



supporting it, being, in fact, firmly anchylosed with the 

 latter. A careful study of the earlier stages of develop- 

 ment shows that only the upper part of this bone is really 

 homologous with the basi-sphenoid, the lower part 

 being the representative of the hinder part of the para- 

 sphenoid. The basi-temporal (Figs. 26 and 27, B.T), as 

 this large membrane bone is called, is firmly anchylosed 

 with the basi-sphenoid, the greater part of the inferior 

 surface of which it completely covers, but is at this period 

 still partially distinct from the representative of the an- 

 terior part of the paraspbenoid (Figs. 26 and 27, Pa.S), 

 the "basi-sphcnoidal rostrum" so characteristic of birds, 

 which is, however, united with the basi-sphenoid. 



1 n front of the depressed basi-sphenoidal region the basis 

 cranii becomes much compressed from side to side, forming 

 a large cartilaginous interorbital septum, the representative 

 of the prepituitary part of the basi-sphenoid and the pre- 

 sphenoid behind, and of the mesethmoid in front. The 

 walls and roof of the brain-case are completed by the 

 squamosals, alisphenoids, parietals, and frontals ; the 

 latter also aftbrding support to the fore part of the base 

 of the brain by means of their extensive in-turned orbital 

 processes. The orbito-sphenoids are altogether absent at 





Fic, 26. — Sectional view of the same. B.T, basi-temporal. 



this stage, but at a later period are represented by two 

 pairs of insignificant ossifications above the postero- 

 superior edge of the presphenoid in the membranous 

 space marked x in Fig. 26. 



A considerable portion of the anterior or ethmoidal 

 part of the interorbital septum is already ossified, forming 

 the lamina pcrpcihliciilaris, or mesethmoid (M. Eth). 



In front of this the cartilage is continued almost to the 

 end of the beak as the scpliim nasi (Fig 26, s. n), or wall 

 between the nasal sacs, the upper margin of which is 

 produced outward into a wing-like expansion, the alinasal 

 cartilage (Fig. 25, Aln) pierced by the external opening 

 of the nostrils (A. N). A further continuation of the same 

 median cartilages is seen in the slender pre-nasal or basi- 

 trabecular (Fig. 27, B. Tr). 



Within the nasal cavity are three pairs of cartilaginous 

 folds, the alinasal turbinals represented by valvular pro- 

 cesses of the ala nasi in some mammals, and the upper 

 and lower turbinals, homologues of the structures bearing 

 the same name in the higher class. The sole representa- 

 tive of the middle turbinal is the flat hinder wall of the 

 ethmoid looking into the orbit, and known as the pars 

 plana (Fig. 25, p. p). , j • - 



There is one more point of importance to be noted with 

 regard to the interorbital septum, namely, the cranio- 

 facial notch (Fig. 26, c.fn\ a natural separation between 

 the epi- and ccrato-trabecular elements, and of great 

 functional importance in the bird, where the beak is move- 

 able upon a sort of J hinge formed by the premaxillse 

 just above this point. 



The membrane .bones of the face are yet to be con- 

 sidered. The premaxilte are large bones partly fused 



A(? 





Fig. 27.— The 



ST- 



Uo-palatine process. 



together in the third line, and provided with well-developed 

 nasal, palatine, and maxillary processes. On either side 

 of the former of these backward projections are situated 

 the nasals, processes from which come downwards and 

 forwards to bound the alinasal cartilage posteriorly. The 

 lacrymal is a largeish bone lying in the upper part of 

 the front wall of the orbit, articulating with the nasal, 

 and directed outwards and backwards. 



The bones of the upper jaw, or palato-maxillary appa- 

 ratus, consist of two sub-parallel series, each of which 

 articulates in front with the premaxilla, and behind with 

 the quadrate ; in the outer series are contained the 

 maxilla, jugal, and quadrato-jugal, in the inner the pala- 

 tine and pterygoid. All the bones in the former category 

 are extremely slender — almost filiform, in fact ; the pala- 

 tines and pterygoids, on the contrary, attain a high degree 

 of development, but neither they nor the maxilla develop 

 palatine plates, the only rudiment of those structures 

 being in the maxillo-palatine processes (Mx.Pa), flat plates 

 of bone proceeding inwards from the maxilla; beneath the 

 palatines to meet the small, single vomer. The palate of 

 the fowl is thus formed on the simplest schizognathous 

 type. 



