OF THE SKULL IN THE HKODELOHS AMPHIBIA. 
539 
All that can be done here is to show what parts grow from, and what parts are 
attached to, these rods. The Axolotl appears to me to be an excellent subject for this 
purpose. 
The fore ends of these bars do not reach quite to the front of the vesicle they embrace, 
and a space equal to their length divides them in front. Looked at in this view, they 
do not suggest the idea of serial relationship to the mandibles. A side view (fig. 3, 
beginning of next stage) lends itself better to such a theory, of their nature*. 
At present there is nothing that can be satisfactorily determined as to qpreoral visceral 
arches ; but the postoral arches present no difficulty. The first of these, the mandibular, 
is very large relatively; it is the principal element that goes to form the gates and 
bars of the face and mouth. The pier of this inverted arch (fig. 1, q.) is still mere 
granular indifferent tissue ; it is an incurved, succulent leaf, with its rounded apex or 
“ pedicle ” directed upwards and inwards, and the front of its base resting upon its free 
inferior segment, the cartilaginous Meckelian rod (mJc.). This stage represents a highly 
modified vertebrate skull, if the apex of the mandibular pier is the next following attach- 
ing point of a “ visceral arch ” to the apex of the trabecular bar. Now it lies a little 
outside the fore part of that bar ; it grows outwards and downwards, and its free bar 
(mJc.) sweeps across the floor of the face, almost at a right angle to the trabeculae. 
The Meckelian rods do not meet by a space equal to a fourth of their length. They 
are stout, sigmoid cartilages, and fit obliquely to the interior angle of their expanded 
pier (q). Of course they lie in a lower plane than the trabeculae ; this is not so evident 
as it should be in the figure. A mass of cells (fig. 2, sp.) in the inside of each rod is 
ready to become the “ splenial ” teeth and bone. 
But this free part of the mandible is seen in the next figure (2) to be the first of a 
series of six pairs of rods. Of these the second pair is a free arch, whose “ pier ” never 
develops in this creature, as in many of the Urodela. The last two never segment 
off a part answering to the lower element of the mandible. This must be made plain 
afterwards, and the difference between these types (in the development of their pleural 
arches) and the Selachians fully explained. 
The sickle-shaped second pair of rods are the hyoid arch (%.)• They are connected 
at the mid line by indifferent tissue, as are all the rods ; no basal piece is chondrified 
as yet, and the hyoid does not acquire one for itself. 
The rest of the bars ( br . 1-4) are feebler and narrower than the hyoid. They are 
* After Professor Huxley had satisfied himself that the whole of the two trabecular rods were pleural, and I had 
found them in the frog distinct from the investing mass, and also, during the cephalic flexure, dipping so as 
to he almost parallel with the primary mandibular rods, the question appeared to be settled. 
For the last two years or more my doubts have been growing as to the truth of this view ; for the trabeculae 
do too much work in skull-building to he mere visceral bars. I find also that Gotte and Baleour doubt or 
deny their pleural nature.- I am inclined to put their “ cornua” into that category, and to regard them as 
undivided representatives of the primary moieties of the vertebral “centra,” and as, naturally, developing an 
undivided neural wall, posterior and anterior, sphenoidal, and ethmoidal. 
4 G 2 
