THE DEVELOPMENT OF BIRDS, ‘ 227 
The skull, in its earlier condition of cartilage and membrane 
only, is thus conditioned :— 
There is a median mass of cartilage which invests the anterior 
end of the notochord and forms the base of the cranial cavity. 
The sides of this investing mass extend upwards and meet 
above, the internal ears being enclosed in these uprising lateral 
cartilages. From the front of the median investing mass, carti- 
lage extends forwards in the direction of the middle of the upper 
mandible, and from this a median and two lateral extensions of 
cartilage mount upwards in the position where we subsequently 
find the median and lateral ethmoid bones. There is also an 
extension upwards from the investing mass on either side, in 
the place where we subsequently find the alisphenoid. From 
the investing mass, four bars of cartilage (on each side) pass 
forwards or downwards. The first of these lays the foundation 
of the pterygoid and palatine ; the second becomes a lateral half 
of the lower mandible. The third and fourth go to construct 
the hyoid—the thyro-hyals being ossifications in the fourth 
descending lateral bar of each side. 
The roof of the cranium is completed by membrane only. 
In this membranous and cartilaginous cranium various 
distinct points of ossification arise and lay the foundation of 
what at first are separate cranial bones. As these bones grow, 
they soon meet together, and their lines of junction are the 
“sutures” of the skull. These still remain pretty distinct in 
a Chicken till it is nearly two months old. 
Only in the young, even of Mammals, can we clearly perceive 
the distinctness of the three bones which together form the con- 
tinuously anchylosed ‘“ periotic capsule,” the names of which 
are : “ prootic,”’ “ epiotic,” and “ opisthotic.” 
The prootic is the most anteriorly situated of the three, and 
shelters that one of the three semicircular canals * of the 
labyrinth which is called the “anterior semicircular canal.” 
It also forms the upper margin of the ‘fenestra ovalis ” 
and the whole of the foramen. through which the auditory 
nerve passes to the ear—namely, the “ meatus auditorius 
internus.” 
The opisthotic is the most inferiorly situated. It forms the 
lower margin of the ‘“ fenestra ovalis,” and entirely surrounds 
the “ fenestra rotunda.” 

* See ante p. 221 
