DEVELOPMENT OP THE SKULL IN THE OSTEICH TEIBE. 117 



but short ; it is well seen within, at its commencement ; and without, at its termi- 

 nation. 



The fourth and last ossific centre to be described, as at present occupying the cranial 

 cartilage, is the basisphenoid (Plate VII. fig. 2, b.s. & r.h.s.) : this bone begins, at first, 

 as an upper and a lower lamina. But unlike the basioccipital, the laminae are not 

 originally in the same vertical line ; for the upper deposit takes place in the fundus 

 of the deep " sella turcica," and the lower appears below the middle of the anterior 

 sphenoidal cartilage (Plate VII. fig. 2). 



This anomaly arises out of a disposition of the cranial structure in the embryo of the 

 Bird, which has, from vicious interpretation, been the cause of much perplexity and 

 error. In fig. 57, F, p. 139 of Professor Huxley's " Elements," a most instructive con- 

 dition of the early skull of the chick is given ; at this stage the occipital region of the 

 skull, with the enclosed " chorda," is more than half the whole length of the basis- 

 cranii; in my earliest ostrich-embryo the cephalic part of the chorda is only one- 

 twentieth part of the length of the skull-base. This change has arisen from the great 

 extension of the investing mass, and of its forthstanding outgrowths, the " trabeculse." 

 In this young ostrich the chorda is separated from the pituitary body by a mass of 

 cartilage equal to that which invests it (Plate VII. figs. 1 & 2) ; whilst the large, oval 

 pituitary space of the earlier stage has been nearly obliterated, not only by the forma- 

 tion of a cartilaginous floor, but also by the convergence, during growth, of the trabe- 

 culse themselves. In an intermediate stage between Professor Huxley's fig. 57, F' and 

 this which I describe, the pituitary space became a mere slit anteriorly ; and this fissure 

 was filled in below by a dagger-shaped band of dense cellular tissue. Whilst the edges 

 of the trabeculse behind were being connected together by a floor of cartilage, the 

 same growth of cells was taking place in the interior of the dagger-shaped band, thus 

 enlarging it, and giving to it a pith of true cartilage. The extension forward of the 

 true pituitary floor reaches in this young ostrich to the middle of the alse nasi (Plate VII. 

 fig. 2, al.n.-r.b.s.) ; and that is the part which answers to the diverging point of the tra- 

 becular cornua in Professor Huxley's figure 57, F' : these cornua were wholly enclosed in 

 the fronto-nasal process [op. cit. fig. 57, F, K). Beneath the " sella" (Plate VII. fig. 1, 

 p.t.s, fig. 2, b.s.) the bony layer is thin ; but the long style or rostrum is already wholly 

 ossified in " Struthio, A ;" this is of great importance to notice, for the style does not 

 chondrify in the cold-blooded ovipara, and in typical birds greater time is allowed for the 

 maturation of the cartilage-cells than in the Struthionidse. The early appearance of the 

 basisphenoid in birds has something to do with its inordinate size ; for the late-appear- 

 ing presphenoid in them never reaches the trabecular region below, the thick pre- 

 sellar part of the basisphenoid uniting with the perpendicular ethmoid ; thus both of 

 these latter bones encroach upon the presphenoidal territory. The median layer of the 

 pituitary floor, and the thick, high, almost perpendicular walls of the " sella" (Plate VIL 

 fig. 1, a.cl.-p.cl.) are as yet unossified, as is nearly the whole remainder of the carti- 

 laginous skull at this stage. 



