DEVELOPMENT OF THE SKIJLL IN THE OSTRICH TRIBE. 147 



signs, such oblique and interwoven knots of nature ; and the journey over it is to be con- 

 stantly made under the uncertain light of the senses, sometimes shining out, sometimes 

 hiding itself, through the forests of experience and particular facts"*. 



The prodigal development of the bird's basisphenoid (Plate XIII. figs. 1, 2, & 3) is 

 seen, not merely in its breadth but also in its length, reaching, as it does, sometimes to 

 the furthest end of the base of the nasal septum ; in its vertical growth also, seeing that 

 it shoots upwards to form a very considerable moiety of the interorbital septum, and in 

 thus joining the great vertical ethmoid, leaves but little territory to the presphenoid. In 

 these generalized birds (the Ostriches), so near to the Eeptiles and to the Mammals, it 

 might have been expected that the basisphenoid would have derived a feebleness on 

 both hands, from its relation to that of these outlying congeners. In one respect it 

 does so, viz. in the symmetrical autogenous basitemporal portion ; not so the azygous 

 piece, or true basisphenoid ; for here in these very birds its transverse growth attains its 

 fulness : every one will see that a free growth of the " investing mass" lies at the root of 

 this matter. 



Compared with the last specimen, the basitemporals [b.t.) are nearer together at the 

 mid line, and do not underlie the basioccipital ; in front of the fossa which separates 

 them the pituitary floor is convex and somewhat perforate. The Eustachian canal 

 Plate XIII. fig. 1, eu.), membranous below, and the deep fissure between the anterior 

 and posterior pterygoid processes, are well seen (Plate XIII. fig. 1, a.p., pr.p.); as also 

 the curve of the great subcylindrical anterior wing, with the somewhat sudden narrow- 

 ing of the prepituitary, high, vertical portion, as below it passes into the rather slender 

 rostrum (r.b.s.), which shows its shallow groove, in the upper view, to receive the trabe- 

 cular structure ; for the rostrum itself is altogether a subtrabecular structure. It would 

 not have been altogether below, but between as well, if the trabeculae had not coalesced 

 together. The oval crown of the high prepituitary mass is the connate anterior clinoid 

 process (Plate XIII. fig. 3, a.cl.), behind which is the transversely oval, deep, backwardly 

 curved "sella turcica" (s.l.t.), into the fundus of which the carotid canals open. 



Behind this pituitary " well," and between the thick crests of the posterior pterygoid 

 processes, is the posterior clinoid bridge (p.cl.) in the " middle trabecular region :" a slit 

 divides it at the middle, and this slit passes down the oblique, gradually narrowing, 

 postpituitary mass (Plate XIII. fig. 3). The embryonic skull explains this ; for after 

 the investing mass has increased around and in front of the pointed end of the noto- 

 chord, it gradually, whilst separating that structure from the pituitary body, grows 

 round this body, the cartilage into which the mass is being metamorphosed meeting 

 behind to form the posterior clinoid wall, and growing up from the edges of the tra- 

 becula in front of the pituitary body to form the anterior clinoid mass. The broad 

 band of tissue behind the " infundibulum " must coalesce more slowly than that in front 

 of it, even in the process of chondrification ; this is repeated when that cartilage is 

 metamorphosed into bone. Whilst the lower lamina of the new basisphenoid is growing 



* Bacon. 



t2 



