956 



OF GENERATION EMBRYONIC DEVELOPMENT. 



those which, as we have just now seen, inclose the Spinal Cord, is however 

 already visible, in the perfect continuity and absence of any trace of seg- 

 mentation in the basal mass. The form assumed by this deposit at a very 

 early period is diagrammatically shown in Fig. 357, c representing the 

 atlas, or last segment of the true vertebral column, formed round the noto- 

 chord, a, which terminates at the point g, where the pituitary body sub- 

 sequently appears. In front of the atlas c, and also, like it, surrounding 

 the uotochord, a large quantity of blastema is laid down, to which is attached 

 anteriorly a pair of rods, g, termed the trabeculse cranii, that separate to in- 

 close the pituitary fossa, and reunite in front of it to form a solid mass, the 

 nasal frontal process. From the margins of this great basal mass of the 

 primordial cranium, a membranous investment rises up to cover the iutra- 

 cranial nervous centres. This membranous cranium, which accurately in- 

 vests the rudimentary brain, is not long persistent, but soon becomes partially 

 converted into cartilage ; the change commencing at the base in the Human 

 foetus at least as early as the second mouth, whilst the upper part retains its 

 membranous condition until a much later period. The primordial cranial 

 axis of Man, therefore, consists of three parts: 1. A membranous roof; 2. 

 Chiefly membranous lateral walls; and 3. A cartilaginous base. The third, 

 or osseous stage of development, is attained by four different processes, 

 namely: 1. By direct ossification of a portion of the cartilage of the pri- 

 mordial cranium ; 2. By part of the cartilage remaining unaltered ; 3. By 

 the disappearance of a small part of the primordial cartilage ; and 4. By 



FIG. 357. 



FIG. 358. 



a- 



FIG. 357. Cartilaginous basis Cranii: a, chorda dorsalis; b, foramen magnum ; c, atlas; d, lateral In- 

 vesting cartilaginous mass; e, auditory capsule; /, pituitary body ; g, trabecula cranii ; h, frontal ir.isul 

 process; i, alse majores; k, alse miuores of the sphenoid bone. 



FIG. 358. Vertical transverse section of anterior portion of the Primordial basis cranii : a, position 

 of vomer; 6, perpendicular lamina of ethmoid ; c, right ala of the cartilage, from which the turbinal 

 bones, e and/, are seen projecting on the opposite side; d, nasal bones. 



the deposition of new bone in the form of opercular, or splint bones, on the 

 exterior of the primordial cranium. The first point of ossification appears 

 immediately in front of the foramen magnum about the ninth week, and is 

 the rudiment of the basilar portion of the occipital bone ; a second (some- 

 times double) point forms in the third month at the forepart of the Sella 

 turcica, and is the rudiment of the basi-sphenoid, or posterior portion of the 

 sphenoid bone. Still more anteriorly, but a little later in point of time, a 

 third double centre of ossification may be seen, which is the commencement 

 of the presphenoid, and in front of this again an ossification commences 

 about the middle of foetal life, which subsequently becoming greatly extended 

 in a vertical direction, forms the perpendicular plate of the ethmoid. With 

 each of these centres of ossification, situated along the floor or base of the 



