OF THE SKULL IN THE UKODELOUS AMPHIBIA. 
583 
In both the notochord is thick and blunt at the apex, and it has a downward curve, 
but not equal to what Mr. Balfour shows in Selachians at the same stage. 
There is no true cartilage at present; but solid rods are formed of dense granular 
tissue, in which a separation of the cells that will form the perichondrium can be seen 
as distinct from the pith, within. 
These rods in the Axolotl are the visceral arches ; in the Frog these with the trabeculae 
in front of them. 
For in the Axolotl the trabeculae are very indistinct tracts of the mesoblast beneath 
the membranous cranium ; but in the Frog they are quite as distinct as the visceral 
rods, and are, indeed, the first pair of the series of rods, and the largest as well. 
This parallelism of the trabeculae with the postoral rods is due to the bend of the 
head upon itself, whereby the floor of the fore and mid brain becomes vertically placed. 
The clefts between the arches are already visible as slits, in the Axolotl, all but the 
first, which never becomes truly open. 
In the Frog, also, it scarcely opens on the outside, although it is a deep sulcus within ; 
but in these embryos the second and following clefts are still mere grooves. 
In the Axolotl the external branchiae on the third, fourth, and fifth bar are large and 
trifoliate ; in the Frog there is merely one small rounded papilla on the face of the 
third and fourth bar (first and second branchial arches). 
In the Axolotl the fossa which is opening to form the mouth is large and transverse ; 
in the Frog it is very small and 4-angled, two angles looking outward and the other two 
fore and aft. 
Second Stage . — In embryos that are straightening, and getting free from their glairy 
envelope, the divarication of the two types goes on increasing. 
In both the head is recovering from its bend ; the clefts are distinct, and the mouth 
open. In the Axolotl the three pair of external gills are very long, but only bifid ; in 
the Frog they are shorter, and the two that are visible are palmate with about eight 
digitiform processes. 
The mouth of the Axolotl is now* a transverse slit with outturned angles ; in the Frog 
it has passed from the lozenge to a square with rounded angles ; and whilst the lips of 
the Urodele are thin, those of the Batrachian are thick and solid, and contain the rudi- 
ments of four cartilages. 
In both kinds the first cleft is imperfectly open, externally ; in both cartilage has 
begun to form in the facial rods ; but whilst in the Frog the trabeculae still appear to 
be the first of the series, in the Axolotl they are not yet chondrified, whilst the visceral 
arches are, all but the suspensorium of the mandible. 
That suspensorium in the Axolotl is as long and thrice the breadth of the small 
trabeculae, which grow like very minute horns, whose base is attached to the sides of 
the apex of the immense notochord. 
But Meckel’s cartilage is already a strong cartilage, sigmoid and transversely 
placed, just bending forwards to the chin; the suspensorium passes outwards and 
