DEVELOPMENT OE THE SKULL OE THE COMMON EKOG. 
153 
but by metamorphosis into fibrous tissue do yield some of the matrix in which the 
parosteal prsemaxillae are formed. The first possible slice that can be taken from the 
fore part of the Tadpole’s face (Plate V. fig. 6, u.d.g.) when seen from its posterior or 
cut surface, shows that the upper horny dentigerous ( Ghimceroid ) plate is formed be- 
tween the upper labials and the outer skin. 
The lower labials (Plate Y. figs. 1-5, and Plate YI. figs. 1-3, l.l.) are drumstick- 
shaped rods of cartilage, having the lower end the stoutest ; the upper end is attached 
to the anterior face of Meckel’s cartilage near the symphysis. Their direction is almost 
vertical; they nearly meet below; and the lower dentigerous plate ( l.d.g ) is rather 
below than behind these rods (Plate VI. fig. 2, l.l , l.d.g). The creases of the lips 
(Plate YI. figs. 1 & 2, l.p.) are covered with small hooked teeth, in addition to the two 
principal plates. The angle at which the lower labial is attached to the Meckelian rod 
(m.lc.) is suggestive of a very different origin for the two cartilages thus unconformably 
related. 
It would appear, from the imbedded condition of the Meckelian rods at this stage, that 
the lower labials are really the effective lower jaws of the Tadpole ; we have in the Ver- 
tebr^ta a procession of three orders of mandibles — namely, the lower labials, the Mecke- 
lian rods, and the ‘ dentary’ “ parostoses.” 
The “ trabecular horns” (Plate V. fig. 1, tr.c.), which sprang from the outer angle of 
the commissure, have grown into long, gently diverging bands, which are strongly de- 
curved where they articulate with the upper labials (u.L). 
These thickish bands are rounded at the edge ; their breadth is best seen in the hori- 
zontal views (Plate V. figs. 3-5). 
They are at present distinct up to the “commissure” (eth., t.c.) (the rudimentary 
ethmoid), which now rises in front of the cranium as a low, rounded, transverse wall. 
This wall lies immediately above the pterygo-palatine bar (fig. 1, gjg .) ; in this view the 
rest of the trabecula is largely hidden by the eyeball (e ) ; that part, however, which 
has coalesced with the next bar is seen behind. 
The sectional view (fig. 2) best shows the manner in which the ethmoidal wall has 
been formed, and how the rhinencephalon (C 1") lies behind its concave face. 
The subcranial part of the trabeculae has nearly the same thickness as the free “ horns ” 
(see edge view of right bar in Plate V. fig. 2) ; nor is their breadth altered in any degree 
(figs. 3 & 4) ; and in this stage it seems difficult to suppose that they do not with these 
horns form one continuous bar with the investing mass ; nothing but a study of their 
development could prevent such a view being taken of them. The trabeculee help 
to form the skull in the same manner as the investing mass; both lie beneath the 
membranous cranium, and both, by upward continuous growth of cartilaginous laminae, 
enclose the fibrous sac. In the postpituitary region this is a very exact repetition 
of the manner in which the neural laminae of the vertebral moieties enclose the “ theca 
vertebralis ;” and the notochordal pith is enveloped by cartilage in the same way as in a 
vertebra. 
ilDCCCL XXL 
T 
