DEVELOPMENT OF THE SKULL OF THE COMMON FOWL. 
779 
The ectosteal sheath of the quadrate (q.) has spread largely ; but the upper and lower 
condyles and the orbital process are still soft. 
The occipito-otic region, when seen from within (Plate LXXXII. fig. 9), shows how 
the superoccipital bony plates spread over the posterior part of the anterior semicir- 
cular canals, and how the “prootic” ectosteal plate has begun to grow over the ampulla 
of the same canal. 
Between the great anterior canal and the edge of the occipital plane, there is a large 
irregular fossa, filled now with gelatinous tissue, and afterwards with ingrowths from the 
parietal and squamosal. The alisphenoidal isthmus (a.s.), and the investing mass ( i.v .), 
where it bounds the “posterior basicranial fontanelle,” have been cut through. The 
left half of the same preparation, seen as an interno-lateral view, is shown in Plate 
LXXXII. fig. 10 ; here, besides a better view of the foramina, we see an additional 
proper periotic bony (ectosteal) plate — the opisthotic (o.jq.) ; two more plates have yet to 
appear — namely, the “ pterotic ” and “ epiotic ” (mastoid). 
The skull at this stage is still further illustrated by a posterior view (Plate LXXXII. 
fig. 8) ; and here the roof-bones are shown. The great upper fontanelle is being oblite- 
rated by the frontals (f.) and the parietals (p.) ; these are formed by an infinity of rapidly 
coalescing bony grains formed in the subcutaneous web : they are flanked by the squamo- 
sals (sq.), which have a similar nature. The two ascending occipital laminae, although 
they have coalesced at the mid line, yet have left traces of their original distinctness in the 
upper and lower notch ; the superoccipitals (s.o.) have not yet invaded this mesial part, 
nor have they reached the ascending edge of the fast-growing exoccipitals ( e.o ). Above 
the selvedge bounding the foramen magnum (fm.) is seen the sinus canal (s.c.) ; and in 
the broad interspace between the super- and exoccipitals the horizontal ( Zi.sc .) and poste- 
rior (jp.sc .) semicircular canals shine out, and are seen to cross each other. Externally, 
and below the exoccipital region (e.o.), the elegant tympanic wings of the occipital carti- 
lage form a thick crescentic selvedge. In Plate LXXXIII. fig. T the postorbital aspect 
of the skull is given;, this is from a less mature specimen than that shown in figs. 1 & 2. 
Here, on each side above the optic nerves (2), and reaching to the olfactory crura (1), 
there is nothing but the laminar presphenoid, for a great extent, except membrane. This 
membrane is becoming bony by extension of the orbital plates of the frontals (f.) ; and 
infero-externally we have the alisphenoids (a.s.), which pass, to some extent, within the 
orbital plates. Much of the alisphenoidal lamina is still cartilaginous ; but the left has 
an irregular ectosteal tract (a.s. 1.) between the inner selvedge and the fenestra (a.sf). 
Another bony tract (a.s. 2) lies above the foramen ovale (5) ; on the right side these two 
plates have coalesced*. In my former paper (Plate vm. fig. 1, p. 125) I showed that 
the alisphenoid (independent of its postfrontal outgrowth) had two bony centres in 
Struthio and some other birds : this appears to be the rule ; but the postfrontal ossicle 
is not present in some of the higher (Passerine) types. 
* This is a reversed figure. 
5 l2 
