584 
ME. W. K. PAEKEE ON THE STEUCTTTEE AND DEVELOPMENT 
downwards at right angles to the trabecula, and has its apex near the fore end of that 
tract. 
So that, in The Axolotl, the trabeculse are wholly incomparable in position to the 
halves of the first visceral arch, instead of seeming, as in the embryo Frog, to be direct 
predecessors or “ serial homologues.” 
In the Axolotl the mandible (Meckel’s cartilage) is chondrified first, before its sus- 
pensory part, and before the trabeculse ; whilst in the Frog this rod appears as a small 
bud, detaching itself from the upper or main part of the arch. 
In both the hind ends of the trabeculse touch the apex of the notochord ; but in the 
Axolotl they lie along it much more, and that part is flattened out more. 
Externally the Axolotl has developed a large free operculum from the hyoid region 
and arch, and this is complete below, as well as at the sides, although of less extent. 
In the Frog this part is very small and only covers the proximal part of the first 
external gill. 
Third Stage . — Embryos that have quite recovered from the cephalic flexure, and in 
which the trabeculse and visceral arches are well chondrified, show a great difference in 
their habit of growth — as great a difference as we should find between the development 
of the embryo of a Bird on the one hand, and of a Mammal on the other. 
In the Axolotl the external gills go on growing, and, developing new papillse, become 
pinnate ; in the Frog they are at their height, and ready to decline, the internal gills 
absorbing them. 
In the Axolotl the mouth is widely gaping, the lower jaw underhung, and the form 
and relative size is like that of the adult Frog ; there are no labial cartilages. 
In the Frog, at this stage, the mouth is small, round, suctorial, and has horny jaws, 
and two pairs of labial cartilages in the closely fitting lips ; there are no bones or den- 
tigerous patches that acquire bone. 
In the mouth and palate of .the Axolotl there are, already, jive pairs of bony, denti- 
gerous plates, namely, the premaxillaries, vomers, palatines, dentaries, and splenials ; 
here, this early appearance of persistent osseous elements is a correlate of the suppression 
of the “ labial cartilages.” 
In the Tadpole of the Frog, which is a sort of temporary Lamprey, the bones spoken 
of do not appear for two or three weeks to come, when the larva is beginning to abort 
its second series of gills and to acquire lungs, and is therefore just passing out of the 
larval state. 
The Axolotl, however, does not, in most individuals , ever cease to be a larva ; now it 
is half an inch in length, but becomes eight inches long or more. 
The chondrocranium is now composed of a number of cartilaginous bars, and these 
bars are developing processes and crests. 
In both kinds there are two pairs of muscular segments below the hind brain, a pair 
on each side of the notochord ; the third pair are cervical ; in neither kind are there 
any “ parachordal cartilages ” behind the investing hind end of the trabeculse, and in 
both kinds these bars are distinct from each other. 
