DEVELOPMENT OF THE SKULL OF THE COMMON FKOG-. 
175 
The “ stapedial,” proper (st.), seen edgewise in Plate VIII. fig. 8", is in shape not un- 
like a mussel-shell (its front portion is shown in Plate VII. fig. 14) ; it is still thin, and not 
much larger than the fenestra ovalis ( f.o .). The figures show these parts from below 
(Plate VII. fig. 14 is a more highly magnified view of part of what is seen in Plate VIII. 
fig. 8"); and it is seen that the bevelled end of the “ medio-stapedial” overlaps the “ inter- 
stapedial,” which in its turn overlaps the “ stapedial.” This takes place to a greater ex- 
tent than in the Seventh Stage, which was a large advance upon the stage before it ; in 
the adult these segments are much more compactly arranged (Plate VIII. fig. 9). 
Tenth Stage. — The adult Frog. 
The structure of the skull in old specimens can now be better understood. I shall 
begin with what first strikes the eye, namely the outer bony plates. 
These have now become fairly specialized into two varieties : — first, those which refuse 
to enter into combination with the endoskeleton, the parostoses ; and secondly, those 
which either directly combine with the cartilage or lie bare upon it, having ossified the 
perichondrium which separated them. Yet in no other group is the difficulty of sepa- 
rating parostoses from ectostoses so great as in the Amphibia*. 
The premaxillaries (Plate IX. figs. 1-3, p.mx.) have not merely increased in size 
since the last stage ; for the nasal processes (n.jox.) are longer and more decurved, and the 
palatal processes (fig. 2, p.ggx.) are delicate and sharp. The maxillaries (mx.) have ac- 
quired a better facial plate (fig. 2>, f.mx .), which articulates largely with the nasals ( n .); and 
the zygomatic process ( z.mx.\ most of which is edentate, reaches to within a short distance 
of the quadrate, and largely overlaps its quaclrato-jugal process; this is a thoroughly 
Batrachian condition ; I am not aware that any other type has this kind of zygoma. 
The “ septo-maxillary” ( s.mx .) is not easy to understand; it appears above as a little 
grain of bone jammed in between the nasal process of the premaxillary and the facial 
plate of the maxillary (figs. 1 & 3, f.mx.) in front of the outer nostril. Feloio , it is seen 
as a little curled spicule of bone in the inner nostril (Plate X. fig. 5, s.mx.). A trans- 
versely vertical section, seen from behind, shows that it is more than half a tube, lining 
the front of the nasal passage and sending down a curled process which can be seen 
from the palate (Plate X. fig. 4, s.mx.). Another section (Plate X. fig. 6, s.mx.) shows 
how it rests upon the nasal cartilage, outside, close behind the little valvular flap ( v.e.n .) 
which partly occludes the external nostril. The nasal (Plate IX. kgs. 1 & 3 ,n.) is largely 
* In my figures I have given an ochreous tint to those hones which become endoskeletal, if even they were 
developed from fibrous tissue at first, and have left those bones uncoloured which, as a rule, continue distinct 
from the endoskeleton of the Vertebrata generally. 
There are, however, four varieties of bone in the adult Frog’s skull, namely : — first, the unmistakable parostoses, 
such as the maxillaries ; second, the loosely applied ectosteal patches, which may or may not become grafted on 
to the cartilage; third, bone-territories which affect the whole thickness of the cartilage, and in which the 
ectostosis and endostosis is nearly synchronous, as the ethmoid, prootie, and exoccipital ; fourth, superficial 
endostosis with no fibrous bone grafted upon it, as in the epiotic and superoccipital regions ; this last is the 
dying out of the chondropterygeous type of endoskeletal bone. 
