DEVELOPMENT OF THE SKULL IN THE OSTRICH TEIBE. 139 



unimportant as to essential nature, but exquisitely fitted to the needs of each of the 

 many myriads of species possessing the vertebrate type of structure *. 



The extreme occipital end of the condyle of the os quadratum in this Emu-chick, 

 most of the metapterygoid process, and a thin stratum of the base of the bone, are stiU 

 unossified ; the clubbed metapterygoid is characteristically struthious (Plate XL fig. 1). 

 In some points the ossific process is more precocious in the Emu than in the Rhea ; in 

 others it is slower. The " articulare" (Plate XI. fig. 6, ar.) is equal already to what we 

 see in the Rhea a week further in advance ; it is developed by an upper (articular) and 

 a posterior lamina (Plate XI. fig. 4, ar.) ; these have already coalesced through the thick- 

 ness of the cartilage ; Meckel's cartilage is much more wasted than in the full-timed 

 Rhea (Plate XI. fig. 5, mk.). The external splints (dentary, surangular, and angular) 

 and the internal (splenial and coronoid) (Plate XI. figs. 1, 4, 5, & 6, d, sa. a., cr., sp.) 

 are in an excellent stage for comparison with those of the Reptile, being exactly in that 

 stage of completeness and yet distinctness which is persistent in the cold-blooded " Sau- 

 ropsida." The anterior part of each dentary forms with its fellow (Plate XI. fig. 5, d.) a 

 beautiful outspread structure, a close counterpart of the next arch but one in front (the 

 intermaxillary) (Plate XI. fig. ?>,px.\ and well illustrating the extreme licence taken by 

 a secondary or splint bone, after having once originated in its simple primordial model. 

 The state of things here, before Meckel's cartilage is removed by absorption, is very 

 similar to the relation existing between the wide-spread growth of the fish's clavicle 

 (coracoid of Owen), and the feeble coraco-scapular cartilage within it (ulna and radius 

 of the same author). The proximal splint of the mandible (the squamosal) (Plate XI. 

 fig. 1, sq.) is still separate (above) from the great posterior sphenoidal operculum (the 

 parietal) [p.), and there is no mistaking its homology with the so-called temporo- 

 mastoid of the frog, and the preoperculum of the fish. 



Above it the parietals have grown upwards to their junction at the sagittal line (Plate 

 XI. fig. 3) ; they are already thick bones ; the frontals (/".) have also met, but the ante- 

 rior fontanelle {fo.) is a lozenge, with sides a line in extent; the posterior fontanelle 

 is filled up by the apex of the superoccipital (s o.). Altogether this opercular roof is 

 more convex than in the Rhea and the Ostrich ; it remains so, though in a less degree, 

 throughout life, thus bringing the Emu near to the Apteryx and the Tinamou in this 

 respect. The original membranous skull is still to seen in the orbito-sphenoidal region ; 

 but the true relation of the frontal splint to the orbito-sphenoidal cartilage (Plate XI. 

 fig. I,/"., o.s.) is well shown at this stage by the manner in which the bony fibres of the 

 orbital plate of the frontal are creeping over the outer surface of their cartilaginous 

 model. Narrowing in front, the frontals run close upon the upper ethmoidal plate, and 

 also passing by it become grooved to receive the pointed ends of the nasals (Plate XL 

 fig. 3). These latter bones (n.) are more ornithic than in the Rhea, having a slight 



• All this is seK-evidont enough to those who have grown weary of working backwards, but which must 

 be repeated again, "line upon line and precept upon precept," to those whose intellects have not been severely- 

 trained by patient labour in the science of development. 



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