DEVELOPMENT OE THE SKULL OF THE COMMON FKOG. 
185 
elegant leafy plates of cartilage ; they both look forwards and are separated by a narrow 
band. The deeply concave anterior margin is accurately semielliptical in outline ; behind 
this great “ notch ” the broad cartilaginous “ basi-hyo-branehial ” plate is continued 
backwards so as to have a length equal to its greatest breadth ; it is narrower behind 
than before, and the sides are concave. 
The single pair of remnants of the first and second branchial arches (hr.) are curiously 
dissimilar; they are not much in front of the “free hypohranchial horns” or thyro-hyals 
( th .) : these latter have become much more slender in the shaft, and are wide behind ; 
they are well ossified by an ectosteal sheath or “ shaft-bone.” 
The great basal plate has irregular patches of “ superficial endostosis ” on its surfaces ; 
but its various outgrowths are soft; it has lost the symmetrical foramina (fig. 1). The 
upper surface of this evenly spread lamina of cartilage is gently concave ; its long sigmoid 
suspensors, the hyoid cornua, give it its exquisite mobility (see Plate IX. fig. 3, hy.), 
a state of things of no little consequence to the Frog with its peculiar mode of respi- 
ration. 
Summary. 
It appears to the writer that several things are proved in the foregoing descriptions 
which could only be guessed at if the subject had not been treated in a somewhat 
exhaustive manner. The most important part of this attempted demonstration of the 
morphology of the Frog’s skull is that which treats of the first two stages ; without these 
the labour would have been greatly in vain, as nothing could have been determined as 
to what is a primary morphological element , and what is a mere secondary structure, 
uniting together and, by that fusion, immediately masking the simple primordial structure. 
But the processes of growth are so rapid, the axis of the embryo is subject to such 
peculiar bending and straightening processes, the relation of parts so greatly altered by 
that which was afar off being brought near, and vice versa , that a complete mastery of 
the earliest differentiation is fundamental to the whole business of research. This 
teaching was long ago impressed upon me by the “ Croonian Lecture,” which contained 
the rudiments of this extended monograph ; I can appreciate its value now that I have 
worked for a long while at the real objects. 
In the skeletal parts I consider that to be a primary morphological organ or element 
which is first differentiated into a definite tract of tissue more solid than the surroundins; 
blastema, and which continues thus distinct until it can be demonstrated to be cartilage 
by the formation of a concentric line which marks off the outer or cortical cells as the 
perichondrial layer. This can be done in the Frog-embryo when it is only one-sixth of 
an inch in length, two or three days before hatching, and when the cephalic and caudal 
extremities project very little beyond the yelk-mass : this is my first stage. This is not 
the point at which to commence general embryological research into the structure of the 
Frog, but is a very convenient stage for the special morphology of the skeletal parts. 
Any opening found at this stage in the walls of the embryo is evidently primary ; for it 
is anterior to that most remarkable dehiscence of the visceral walls by which the “ facial 
mdccclxxi. 2 c 
