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MR, W. K. PARKER OK THE STRUCTURE AXD 
clefts ” are formed ; there is at this stage an opening of this kind, a visceral font anelle ; it 
is the opening of the mouth (Plate III. figs. 1 & 8, m.). The secondary openings ( clefts ) 
are of the utmost interest, as the two foremost of them are persistent in the highest 
Vertebrata, and form the passages necessary to the organs of smell and hearing. 
The primordial cartilaginous bars which are so definitely separated by the clefts are 
modified to an unlooked-for extent themselves, and also serve as a foundation upon 
which, continuously, structures are built with which they could have had no primary 
relation. 
The Cranium , 
in the first place, may be remarked upon as being largely constructed upon facial bars, 
as well as having a true axial region, continuous with the vertebral column. 
The neural axis, whilst passing from a grooved into a tubular form, acquires a mem- 
branous investment ; this investment lies immediately upon the notochord and the sym- 
metrical vertebral rudiments. In the vertebral part of the axis this is a mere cylinder ; 
but in the cephalic region, where the vertebral rudiments are continuous to the end of 
the notochord, the neural mass becomes swollen into vesicles, and the membranous 
investment into pouches. This lobular bag, with its cerebral contents, does not 
derive the skull, which more strongly encloses it, from the continuation of the axis 
merely ; for the axial part stops short behind the down-turned pituitary vesicle (Plate 
III. fig. 4). 
The remainder of the skull has a facial foundation; it is built upon the first pair of 
facial arches, which straighten themselves beneath the membranous cranium, and then 
send outgrowths of cartilage upwards which form walls on each side and in front. 
Not only so, but the auditory sacs imbed themselves in the lateral walls, which are 
really derived from the axis, thus forming a large “ fenestra ” on each side, which, 
however, is closed by the auditory sac. 
Thus it is seen that the skull of the Frog is a morphologically compound structure. 
The Sense-capsules. 
Two pairs of these, the optic and the auditory, have their own cartilaginous pouch, 
and are, primarily, equally independent of all the surrounding skeletal structures, the 
earhall as much as the eyeball. 
The olfactory sacs, however, are entirely membranous ; and the labyrinthic chambers 
in which they are lodged, and to which they form a lining, are developed independently 
as superstructures upon the first pair of visceral arches, and upon the secondary “ con- 
nective” which binds them together. 
If all this be true, then the terms cranium and face are commonly used in a very arbi- 
trary manner; the terms axial, visceral, and sensory must be used as much as possible 
if we would speak correctly of these things. 
But the term skull for the cartilaginous or bony box enclosing the whole brain can 
