| OCR Text |
Show 12f) LECl'URE VIII. ON ~rHE S~rRUCTUliE OF THE SKULL. 'rHE DEVELOPMENT OF 1'HE IIUl\1AN SKULL. As might be expected from the nature of the ca. e, it J1as not yet been po ·ible to oLtain a eri of hunw,n 1nbryo~ , in every stage of dev lopment, sufficiently larg to enabl embryologists to work out all the details of the formation of th :\ lnnnan skull. But all higher vertebrate e1nbryos so nearly follow one and the same type of early developmental Inodi:fication, that we 1nay reason, with perfect confid nee, frmn the analogy of the lower Vertebrates to man, aud fill up the blanks of our ob" .r vations of human embryos by investigations of the chi ·k, the dog, the rabbit, or the pig. In the chick,* the fir t indication of the body of the embryo js an elongated, elevated area f the blastod rill, the axis of which is traversed by a lin ar groove. The one e11d of the elongated area is wid r and 1nor di tin ·tly rai d up from the rest of the Llastoderm, than thP other : it i th cephalic end (Fig. 31, A, a), and the linear groove stops , hort of tho ronndud extremity of thjs part of the levat cl ar a. A peculiar cellular cylinder, tapering off at each nd, the noto ·horcl, i soon discerned occupying the bottom of this groove, b neath the outer, serous, or neuro-epidermic layer of the germ. A laminar outgrowth of th convex sum1nits of tho ridges which bound the primitive groove now tak s plac , jn that part of the embryo, which will ev ntually b con1 th middle region * Sec Lcetun' IV., pp. G4- 6G. THE DEVELOPMEN'l' OF THE FOWI,'S SKUJ,L. ]27 of the head; and tho dorsal laminm, thus produced, ext nding forwards and backwards, like parapets, upon each side of the primitive groove, lay the foundations of the lateral walls, not only of the skull, but of the spinal column. Very early, however, the boundary line between skull and Rpinal column is laid down, by the appearance in the substance of the bases of the dorsallamince and the adjacent midclle layer of tho blastoderm, of the first pair of those quadrate masses of condensed tissue, the proto-vertebrm (" Urwirbel" of the Gennan writers), which are the foundations, not only of the bodies of the vertebrce, but of the spinal muscles and ganglia. The proto-vertebrre increase in number frOin before backwards; and, at length, extend through the whole range of the spinal column, while none ever make their appearance in the reo-ion which will be converted into the skull. b The edges of the dorsal lamince now unite, the coalescence taking place first in the middle cephalic region, and extending thonce backwards and forwards ; at the same time, the cephalic canal becomes separated into three distinct dilatations, or cerebral vesicles, of which the anterior is by far the most marked (Fig. 57, A, I, II, III). . Tl~e rudimentary cranial cavity next becomes bent upon Itself 1n such a manner, that the longitudinal axis of the first cerebral vesicle takes a direction at right angles to the axis of the third, and. of the spinal canal generally. In consequence of this change, the 1niddle cerebral vesicle occupies the summit of the angulation, and becomes the most anterior point of the 'rvhole body (Fig. 57, 0, D). The bend thus produced is the cranial flexure. It results in the division of the floor of the cranial cavity into two parts, an anterior and a posterior, which are at right angles to one another (Fig. 57, 0, D, E). Hitherto, no trace of the notochord has been observed in the anterior division, that structure ending in a point behind the flexure (Fig. 57, D, E, h). As development proceeds, the anterior cerebral vesicle becomes divided into two portions,-an anterior, the vesicle of the cerebral hemispheres (P') ; and a posterior, the vesicle of the thu·d ventricle (I ). In the upper wall of the vesicle of the third |
