1 
296 ME. W. K. PAEKER ON THE STETJCTUEE 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. 6, 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, G 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 hilus of the kidney-shaped third cerebral vesicle, is the “third or median trabecula” 
of Patiike — 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 which 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 
* Rathke erred in supposing the “paired rafters,” or symmetrical trabecuhe, to be outgrowths of the 
investing mass of the notochord. 
