OE THE SKULL IN LEPIDOSTEUS OSSEUS. 
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First Stage.—Recently hatched young of Lepidosteus, 2|- to 4-|- lines long. 
At this stage (Plate 30, figs. 1, 2) the mesocephalic flexure is at its fullest develop¬ 
ment, but the angle formed by the meeting of the 'para- chordal and pro- chordal regions 
is very open. Moreover, the bend appears less than it is, for the snout is very large 
and dilated, and its axis is coincident with that of the cerebral hemispheres (C la .) and 
of the mid brain (C 2 .). 
The fissure, however, between the fore and hind brain (C 3 .), and below the mid 
brain, is very large, and is turned backwards considerably, above ; the notochord ( nc .) 
seeks to ascend into this space, but only rises one-third of the way to the top ; the 
third nerve descends in. this cavity to reach the orbital muscles. 
In the sectional figures given to illustrate this stage, very little of the primordial 
cranium comes into view; it is not yet formed into consistent cartilage, but the 
‘ £ embryonic cartilage ” is sufficiently developed to show the cranial and facial 
rudiments. 
The pituitary body (py.) is distinguishable from the palatal skin, and it is quite free 
above, the infundibulum (inf.) lying over it. 
In both figures (Plate 30, figs. 1 and 2, cl 1 - 4 .) the clefts and visceral folds are visible on 
their inner or hypoblastic aspect; in the more advanced specimen a rod of solidifying 
tissue was seen as cut through in six of these, namely, the mandible, hyoid, and four of 
the branchial arches (mn., hy., hr 1 ' 4 .) —all but the fifth , which remains rudimentary. 
The heart (h.) is seen impacted between the mandibular rudiment (mn.) and the 
yolk (y.) ; it thus lies as far forwards as the snout. 
The notochord (nc.) is very large, and in the more advanced specimen (Plate 30, 
fig. 2) the terminal hook of this cephalic end of the rod is fixed in between the 
infundibulum (inf.) and the swelling antero-inferior part of the hind brain (C 3 .). 
Second Stage.—Recently hatched young of Lepidosteus, 5 to 5^ lines long. 
In somewhat larger specimens (about 11 millims. long) I was able to make prepara¬ 
tions of the whole chondrocranium (Plate 30, fig. 3) : a sectional view is given of a 
somewhat larger specimen (Plate 30, fig. 4). 
The section shows that the skull is rapidly straightening, but of course the great 
clinoid fissure is never obliterated. This is formed by the primordial flexure, and is 
always permanent in the Vertebrata; but the flexure itself is not so great in this type 
as in the Amphibia and the “ AmniotaP 
The notochord (Plate 30, fig. 4, nc) is not traceable so far up in the clinoid region as 
it was; the pituitary body (py.) is still independent of the infundibulum (inf). 
In the dissected skull (Plate 30, fig. 3) the basal and facial parts alone are chondrified, 
the sides and roof are entirely membranous ; the cartilage is still very fragile, but 
quite differentiated from the surrounding connective-fibre cells. The chondrocranium 
3 M 2 
