DEVELOPMENT OF THE SKULL IN THE LACERTILIA. 
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brain, for the oral involution (see also fig. 3) is very imperfect, and the pituitary fold 
can scarcely be said to exist as yet.'"' 
The mesocephalic flexure produces results not easy to be understood. Rathke’s 
“ middle trabecula,” with the apex of the notochord lying in it, gives the appearance 
as if the axis of the embryo were striving to grow through the head, between the first 
and second vesicles of the brain (Plate 37, fig. 3, m.tr. ; and also “ Pig’s Skull,” Plate 28, 
fig. 6 ; and Balfour’s c Elasmobranchs,’ plate 14, figs. 1, 2, m.b.). 
The mid brain is horseshoe-shaped, and the concavity is filled with stroma, in which 
the notochord lies ; afterwards, the cartilage of the base (parachordals and trabeculae) 
grow up in this direction behind the pituitary body, as the “posterior clinoid” wall. 
That wall, therefore, is the permanent mark or “ stigma” of the mesocephalic 
flexure ; but the head, recovering its straightness to some extent, always brings the 
trabeculae into the same line as the parachordals (post-pituitary region of base of 
cranium), but they were at one time at right angles with each other. (See Plate 39, 
fig. 3.) 
That flexure, the pituitary and pineal bodies, the “visceral,” as distinct and different 
from the costal arches, the fusion of segments in the head, and consequent abortive 
modification of certain cranial nerves, and the suppression (apparently) of several pre¬ 
oral arches ;—the meaning of all these things will be revealed when we know what 
kind of creature an archaic entomo-cranial Vertebrate was. 
Fifth Stage. Half-ripe Embryos of Lacerta agilis, with head -y inch long. 
To harmonize the structural conditions just described with those given first in the 
skull of the adult Lizard would be impossible without the intervention of a stage like 
this, in which the metamorphosis is well nigh complete, but has left various infantile 
marks and features. 
The outer form of the head (Plate 38, figs. 5-7), like that of a somewhat less mature 
embryo of the Snake (“Snake’s Skull,” Plate 28), suggests the idea of some “old Dragon’ 
—some Liassic or even Carboniferous “ Leviathan”—that had broken bounds from the 
great Fish-territory, and had become a true “ Sauropsidan,” developed with all the 
embryonic membranes, and yet stamped with the Fish’s brand on every part. 
This half-ripe embryo has huge eyeballs, which would seem to indicate an Ichtliy- 
opsaurian descent; yet it is scarcely conceivable that those large-eyed giants could 
beget and conceive the hosts of modern Lizards, many of them scarcely larger than 
Hornets and Dragon-flies ; this is “a question to be asked” of the Evolutionist, which 
may possibly receive an answer some day. 
When we know what exists in the perfect form of the mature Lizard, then we see 
* I believe that my figures of these parts in my first paper on the Development of the Skull in the 
Batrachia (“ Frog’s Skull,” Plate 3, figs. 4 and 12, and Plate 4, fig. 8) are open to the same criticism. I 
shall be glad, at any cost, to atone for these errors, my apology being the difficulty of the work itself, and 
the loneliness of the worker. 
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