OF THE SKULL IN THE URODELOUS AMPHIBIA. 
545 
figure, the lower part of which reaches the membranous inner roof of the labyrinth. 
This is evidently the primary involution or “ aqueduct.” 
The mouth-floor shows sections of the same parts as in the last. 
The eighth section (Plate 21. fig. 9), without the floor, is a front view of a section 
with some thickness, showing the back of the ear-sac cavity and the thick part of the 
notochord, behind the trabeculae (: nc ., m.s.). Here there is only muscle and connective 
tissue on each side of the notochord. The hyoid ( c.hy .) has been severed near its apex. 
The ninth section (Plate 21. fig. 10) is a back view of another section taken between 
the ear-sacs ( au .), and showing the ganglia of the 9th and 10th nerves (9, 10). 
The front of this section belongs to the back of the eighth. Here the actual apices of 
the hyoid cornua {c.hy.) are caught, but the branchials are not figured. These hind 
sections explain the two main figures (Plate 23. figs. 1, 2). 
Fourth Stage. Young Larvae f of an inch long. 
In this stage the “ fry ” were nearly twice the length of the last, so that ample time 
had elapsed for very important structural modifications to appear. 
In this stage the pituitary body (Plate 22. figs. 4, 5 ,py.) is halfway between the frontal 
wall and the occipital condyles ; and the cephalic notochord is as long as the prepituitary 
part of the skull. The front half of the chorda is a long cone with an obtuse apex ; the 
hind half is cylindrical, slightly inclined to the hour-glass shape. There are two equal 
parachordal tracts of cartilage ; those which embrace the fore part of the notochord are 
the slabs of early cartilage that form the postpituitary region of the trabeculae ( tr .) ; the 
late , newly-developed cartilages are the moieties of the investing mass, the hinder or 
Huxleyan “ parachordals.” These latter are sickle-shaped tracts, whose ends curve 
outwards away from the notochord. The fore part is sharp, and wedges in between the 
trabecular plate and the ear-capsule. The hind part is knobbed ; it is separated from 
the ear-sac by the ganglion of the 9th and 10th nerves, and is the rudiment of the occi- 
pital condyles. The membranous tract separating these two pairs of cartilage is the 
primordial boundary-line between the basioccipital and basisphenoidal regions. It is 
represented in the Mammal, afterwards, when ossification has set in, by the “ spheno- 
occipital synchondrosis.” 
This primordial landmark, then, separates the occipital from the posterior sphenoidal 
regions. Its development is very late in the Amphibia. The two tracts appeared to 
be quite contemporaneous in their chondrification in the Salmon (“ Salmon’s Skull,” 
plates 1 & 2). They were separate, even in the fifth stage, in that type (plate 4. 
figs. 2 & 3, tr., iv.) in “fry” the second week after hatching; but the ends of the tra- 
beculae, which overlie the hinder tract (iv.), had lost the retiring notochord. In the 
early stages (plates .1 & 2) they scarcely embraced it at all. 
In the Fowl (“ Fowl’s Skull,” plates 81 & 82, lg., iv.) the fore end of the investing 
mass is turned outwards and is truncate, the trabecular apices retire outwards from the 
notochordal apex, which also retires backwards at the same time. 
MDCCCLXXVII, 4 H 
