DEVELOPMENT OE THE SKULL OF THE COMMON FKOG-. 
149 
“ quadrate.” There is a part, however, which is not represented by the metapterygoid of 
the teleostean fish ; this is the connective band, which has now attached it to the “ invest- 
ing mass,” and which I shall describe anon. In the last stage we saw (Plate III. fig. 1) that 
the first and second postorals were diverging below; that divergence is now very great 
(fig. 7), and its meaning is self-evident. With the most perfect fidelity to the ichthyic 
type, the auditory capsule is now overshadowing the apex of the second poststomal, 
ready to coalesce with it ; and this bar has now begun a series of metamorphic changes 
that yield in interest to nothing that the morphologist encounters. It has become divided 
into two nearly equal parts ; and the lower piece diverges backwards so far as to leave a 
large space between it and the “ quadrate region.” The upper piece is the “ hyo-mandi- 
bular,” the lower the “stylo-cerato-hyal;” the space is preparing to become the “tympano- 
eustachian cavity and the small projection of cartilage below the new condyloid hinge 
is a rudiment of the “ symplectic” in a plagiostomous condition. The hyo-mandibular 
piece and the symplectic bud cleave close to the preceding bar, save at the top : all this 
will be seen to be full of meaning in subsequent stages. The four next, diminishing, 
faucial arches, the branchials, have not altered much ; but they are more perfect in form, 
and are still quite distinct from each other and from surrounding organs. Two other 
cartilages are seen in this side view, namely the upper and lower labials (Plate IV. fig. 
7 , u.l., l.l.); these are to be seen in the other figures. Much as a lateral view dis- 
plays, there is also much to be learned from vertical and horizontal views. In fig. 9 the 
membranous cranium and its contents have been removed, all but the diverticulum, 
which contains the pituitary body (p.y.) ; the cephalic part of the notochord and the “ in- 
vesting mass” ( ri.c ., i.v.) are shown as horizontally cut through ; all the rest of this view 
is facial. In this view, a solid object seen from above, the upper labial cartilage (u.l.), 
is seen to be divided into a right and a left piece ; these pieces only half fill the thickness 
of the upper lip, which is filled with gelatinous tissue anteriorly. The middle of the 
fore part of the palate has been cut away, exposing the oral cavity (m.). 
Within three or four days, whilst the embryo has grown a line more in length, the 
facial arches have become largely confluent. The trabeculae have not only grown be- 
neath the skull more perfectly, and become fused to the mandibular pier above and 
tied to it below by the pterygo-palatine bar, but they have also formed a large commis- 
sure in front of the pituitary body*. 
This palato-facial balk is the common support and foundation, indeed, of all that is 
termed ethmoidal in the higher types ; yet at the mid line it is a secondary structure. If 
we compare this horizontal view of the trabeculae with that of the previous stage (fig. 5), 
we shall see how rapid have been the changes, not the least remarkable of which is this 
closing-in of the “ palatal bands” (Gaumenleisten) round the pituitary body, which in the 
earlier conditions (see the position of the pituitary body,^ry., in relation to them in fig. 5) 
This is termed by Muller “vordere Commissur der Gaumenleisten unter der Nasencapsel ” in the Ammo- 
ccetine stage of the Lamprey ; and in the adult it forms his “ knocherner Gaumen, or hard palate” (‘ Myxinoids,’ 
pi. 4. fig. 3, H, and fig. 7 6 , d'). 
