116 
ME. M. K. PAEKEE OX THE STEUCTUEE AND 
fauces between the infoldings and thickenings that have formed beneath the brain. 
The pituitary body {py.) lies between and almost upon the converging mandibular rod 
(v. 3), and these terminate above in a transverse line a little behind the pointed end of 
the notochord (nc.). An equal space exists in front of the pituitary body, between it, 
behind, and in the membranous cranium and subocular bar in front : there is no space 
between the trabecula and the cranial sac ; it cleaves fast to the skull from the first. 
Several minor stages and various conditions of the mandibular arch (third facial, first 
postoral) are given ; perhaps the most important is my very earliest (Plate I. fig. 1, v. 3). 
Here it is but a granular tube filled with and enclosing fluid stroma, and yet it 
has three most important regions marked out. The first of these is the slender “ meta- 
pterygoid” band, running up and curving inwards as far as the next arch: this part has 
its normal position under the fore edge of the auditory capsule. 
The next region is the middle, the swelling part, which shows the commencement of 
the “ orbitar process,” here, at the beginning, having a position which it only acquires in 
the fifth or metamorphic stage of the Prog (see “Frog’s Skull,” Plate v. fig. 1, and Plate 
vii. fig. 1, or.p.). The third region is the MecJcelian ; it becomes afterwards a free seg- 
ment, the mandible ; it is clubbed at its end as in the Tadpole, but it lies as yet some 
distance from its fellow. I 11 more advanced embryos (Plate I. figs. 3, 7, 9, v. 3) the first 
postoral has grown very much ; it is very variable as to length, even at exactly corre- 
sponding stages, and the clubbed Meckelian end has spread into a flat spatulate expan- 
sion (Plate I. fig. 7, and Plate II. fig. 1, mn.). This bar is not dissected out in figs. 3, 
6, & 9, but in fig. 7 we see its uttermost development as a simple tape-like band of 
cartilage, its flat apex and “orbitar process” turned inwards beneath the front of the 
auditory sac, and its Meckelian region converging to its fellow in front, and flattening- 
out at the symphysis. The apex, with its “orbitar” expansion is well shown with its 
relation both to eye- and ear-ball in fig. 5 ( v . 3, mn.). 
The first clear differencing and formation of the second postoral (hyoid) is shown in 
fig. 1 (v. 4 ) ; it has an enlarged and incurved apex, and is thickened in the middle and 
below : the umbilical vesicle ( u.v .) reaches the mid line between the two bars. This condi- 
tion lasts but a short time ; for in embryos but a little more advanced (figs. 3, 7, 8, 9) the 
part of the blastoderm enclosing the yelk has retired so as to expose the under surface 
of the lingual region, and even the opercular fold of the second postoral bar ; moreover 
the ear-sac (au.) has still its great lipped opening. Now, but not before, the first postoral 
cleft (fig. 3, cl. 3 ) is very distinct, and the top of the long hyoidean band of cartilage 
largely curls itself under the auditory sac (figs. 3, 5, 7, 10, v. 4, cm). Also we now 
find the first of a series of azygous cartilages, which are denied to the three front arches ; 
this is the “ glosso-hyal” or true “basihyal” piece (figs. 7, 8, g.h.). But, further, the 
first two postorals would he remarkably alike, save that the apex of the second is 
broader and more developed even now than that of the first, so early does each arch 
show the marks of its hereditary modification. As all the arches have a tendency to 
project forward, this keystone piece stands mostly in front of the lateral bars ; it will be 
