AND DEVELOPMENT OF THE BIRD'S SKULL. 103 



head that had begun to straighten, and nearly comes up to the earliest part of my second 

 stage (" Fowl's Skull," plate lxxxi. fig. 3). 



The interorbital septum (pe) has here been cut through in the anterior sphenoidal 

 region, and, as in the Mammal, the sphenoidal sinuses (sps) are exposed ; they have not, 

 however, any retral continuation of the nasal wall in a cartilaginous state, and they are 

 very narrow. Below, the strongly compressed trabeculse (tr) have coalesced, and are now 

 composed of hyaline cartilage ; although the commissure is finished below, the crests of 

 the trabecule are distinct. These solidify more tardily ; and their conversion into true 

 cartilage taking place at the same time as the cranial floor with its double descending 

 keel, there is at present no line of demarcation between these regions. 



Afterward (see " Fowl's Skull," plate lxxxiii. figs. 4, 5, & 11, p.s, o.s) the mem- 

 branous interorbital space (i.o.s) redivides these dissimilar tracts, leaving for some 

 extent a descending presphenoidal keel and an ascending trabecular crest. 



The orbito-sphenoids (figs. 3 & 4, os) are small, and lessen rather than increase in size. 

 The palatines are here narrowing towards the pterygoid region. 



The next section (fig. 5) is through the optic lobes (C 2 ), the pituitary body (py), and 

 the back part of the eyeball. 



The manner in which the face is tucked under the head at this stage makes a most 

 remarkable display of parts possible in one section : these are the apices of the tra- 

 becular (tr) and pterygo-palatine arches {pg), and at the same time the distal extremities 

 of the first and second postorals (mk, ch, b.hy). The internal carotid arteries (ic) are 

 seen piercing the fibrous pituitary floor mesiad of the ends of the 'trabecule, the 

 actual apices of which look downwards in the figure. On each side of the high and 

 narrow mouth-roof (m) are seen nearly the whole of the pterygoids (pg) ; their " trabecular 

 process," or epipterygoid apex, looks a little inwards below, and is seen enclosed in the 

 rudiment of a " soft palate." 



The end of each " sphenoidal sinus " (sps) is here seen to reach to the side of the sella 

 turcica. The floor of the mouth (m) contains laterally the Meckelian rods (mk), supra- 

 mesially the tips of the small distal cerato-hyals (ch), and below the basi-hyal (b.hy). 

 Behind the optic lobes and eyeballs the head has become very narrow. The next section 

 (fig. 6), which has been made through the fore part of the auditory capsules, shows 

 this. 



The notochord (tic) lies between the two halves of the investing mass (iv), which has 

 completely coalesced with the periotic cartilage. This section shows the cochlear cavities 

 (cl) and those of the anterior semicircular canals (asc) with their ampullae. Below the 

 cochleae are seen the internal carotids (ic), and towards the mid line the submucous 

 tissue is of great thickness ; it forms the nidus in which those remarkable ornithic 

 detachments of the parasphenoid, the basitemporals (bt), are developed. All but the 

 fore ends of the mandibular rods are here shown, and also their piers, the quadrates ; 

 mesiad of these are seen the basi- and cerato-hyals (bhy, chy). 



With regard to the parasphenoid, although it is developed as an azygous bone ante- 

 riorly, yet the thick bed of stroma which forms its mother-substance is a symmetrical 



