DEVELOPMENT OF THE SKULL IN THE OSTEICH TEIBE. 119 



being continuous with the septum of the orbits and the nose, and also with each 

 other. 



They may, however, be described regionally ; and then we have the aliethmoid 

 (al.e., eth.), which grows outwards and downwards to form the feeble upper and middle 

 turbinals, the aliseptal plates (al.s.), which form the inferior turbinals, and the alinasal 

 laminae (al.n.), the homologues of our alse nasi. I shall describe these parts more fully 

 in embryos further advanced. The prenasal cartilaginous rostrum (p.n.) belongs to 

 the visceral laminge of the front, or terminal part of the head ; in this stage it is at its 

 fullest growth, and soon begin to shrink, but not before it has served as a model to the 

 splint bones which are vicarious of it. The great general fontanelle (Plate VII. tig. 1, 

 o.s.-s.o.), or space left unfinished by cartilage — the orbito-sphenoids, alisphenoids, 

 periotic crest, and . superoccipital cartilage together failing to form even half of the 

 brain-case — has to be roofed in by the frontals, or orbito-sphenoidal splints, and by the 

 parietals, or alisphenoidal splints (Plate VII. figs. 3 & 5,f.p.). These two pairs of bones 

 are formed, as usual, in the outer layer of the embryonic " dura mater;" at this stage they 

 are mere patches, not half the required size ; they are moreover mere aggregations of 

 minute, irregular bony points, only partially hiding the mat of fibrous tissue in which 

 they are being deposited. The superoccipital cartilage (s.o.) has here no splint or 

 interparietal piece as in the mammal, where two such ossific centres appear above the 

 superoccipital and between the parietals. The mesencephalic region (Plate VII. 

 figs. 3, 5 & Q,fo.) is still very prominent, although the optic lobes are beginning to be 

 inferior to the hemispheres in size. 



Further forward there are two other pairs of feeble bony patches, the splints belonging 

 to the olfactory laminae ; they are the external and the upper ethmoid splints, tlie nasals 

 and the lacrymals (Plate VII. figs. 3 & 5, n.l.). 



Their present form is a small rough model of their persistent condition ; they have 

 been well differentiated in mere connective or fibrous tissue ; and at this stage the lime is 

 merely diffused through the web in small granules : such a condition, the fibrous web 

 being rendered denser by so much lime, might easily cause these tracts to be mistaken 

 for true cartilage, if only low powers were used in their examination. The nasals and 

 lacrymals certainly belong to the facial category, for they are opercular additions to the 

 olfactory cartilages, the upper surface of which would be a continuation of the cranial 

 Jloor, if the brain were continued so far forward into the nasal region. 



Before leaving the skull- and sense-capsules, I would remark that in the formation of 

 true hyaline cartilage the cells of the orbitonasal septum do not bear a Jilial relation 

 to those of the " trabeculse." Each structure is preformed in simple or indifferent 

 tissue, and the differentiation of tracts of such cellular substance into a fibrous web, or 

 into cheese-like cartilage, is one of the first great formative processes of which we can 

 give no account. Afterwards the growth of each region takes place by the proliferation 

 of such metamorphosed cells, the progeny formed within the parent cells being of the 

 same nature as their parents. 



