296 



ME. W. K. PAEKEE ON THE STEUCTUEE AND 



this becomes the "basihyal" of anthropotomy, but answers to the first " basibranchial 

 of the Fish, These and the other "conjugations" will be shown in the more advanced 

 stages. 



The Notochordal Region and Membranous Cranium. — With the arrest of the soma- 

 tomic divisions in the cephalic region of the embryo, and the great modification of the 

 nerves of common sensation and of motion, we have no certain guide as to how much or 

 how little of the spine the notochordal region corresponds to. The notochord retreating, 

 relatively, from the fore end of the investing mass and becoming in time the temporary 

 axis of a single basal bone, the basioccipital, although it gives a vertebral character to 

 its own territory, is yet placed by its altered conditions in a new category. 



In my first stage I take the skull when it has been fully bent upon itself — the 

 " mesocephalic flexure ;" and at this time the large notochord (Plate XXVIII. fig. %,nc.) 

 bends suddenly upwards, and ends in a free blunted point, exactly opposite that infolding 

 of the membranous cranium which partially severs the second from the third cerebral 

 vesicle (C 2, C 3). The investing mass stops short of the apex of the notochord and 

 lies beneath its plane. The relation of the two, as seen from above, is given in the hori- 

 zontal view (Plate XXVIII. fig. 8), and as seen from below, diagrammatically, in fig. 5. 

 The vertical section (Plate XXVIII. fig. 6) shows the notochord covered above with the 

 membranous cranium (dura mater and cells of the cutis), and bearing in its hollow 

 the medulla oblongata [m.ob.) and the vesicular cerebellum (C 3). The three structures 

 here seen behind the pituitary body {py.) form the primordial " posterior clinoid wall ;" 

 and the rounded mass of delicate gelatinous stroma which lies above these three parts, in 

 the hihts of the kidney-shaped third cerebral vesicle, is the "third or median trabecula" 

 of Rathke — a structure quite temporary, as that excellent author averred, and of no 

 morphological import*. 



Only in the basal region is there at present any developed hyaline cartilage (Plate 

 XXVIII. figs. 5, 6, 8, and Plate XXIX. figs. 4 & 7, i.v.), although it does appear in 

 large tracts afterwards infero-laterally, and even above also in the occipital region. At 

 present all but the notochordal region of the cranium is a very soft and delicate mem- 

 brane, inclosing the large blebs into Avhich the great neural axis has developed. After- 

 wards this membrane will in certain parts split up into three strata — the dura mater 

 within, the granular territories in which the " investing bones " develop will lie on 

 the outside, and in the middle the hyaline cartilage of the occipital and sphenoidal 

 regions. At present the skin is represented, but not thickened into distinguishable 

 dermis, over the third vesicle (Plate XXVIII. figs. 1, 2,*& 6, C 3) ; afterwards this vesicle 

 will be entirely enringed behind, in the manner of a vertebra, the middle layer of the 

 membrane chondrifying directly upwards from the investing mass. But in the basi- 

 sphenoidal region only as much cartilage as was primarily related to the free end of the 

 notochord (namely, the " postclinoid wall") has any remnant in it comparable to a 



* IIatiike erred in supposing the "paired rafters," or symmetrical trabeculae, to be outgrowths of the 

 investing mass of the notochord. 



