DEVELOPMENT OP THE SKULL OF THE COMMON FOWL. 
791 
Seventh Stage . — Chicken 2 Months old. 
In this much more advanced condition of the skull many of the sutures still remain, 
the two new bones described in the last stage have already lost their distinctness, and 
two new pahs of osseous centres have appeared. 
In Plate LXXXV. fig. 1, the left half of the skull is shown from the inside, and 
the orbital septum, not touched by the saw, is shown on its right side ; this figure is mag- 
nified 3 diameters. 
Altogether the structures have begun to acquire that massiveness which characterizes 
the skull of the Fowl equally with that of the Ostrich. The delicately toothed sutures 
above, between the roof-bones, and the narrow synchondrosial tracts in the primordial 
skull, make this a very fitting stage for comparison with the skull of a Reptile. 
The notochord is now fairly vanished from the basioccipital (b.o.) ; but the vertical 
dimple in the condyle is a permanent remembrancer of what once existed ; the bony 
mass which has swallowed it up and replaced it is now a massive lozenge, capped with 
cartilage behind. It is surmounted on each side by the exoccipital ( e.o .), the smooth- 
faced wedge-like opisthotic (op.), and by the postero-inferior margin of the many-sided 
prootic {'pro.). Crowning all these, there is the large elegantly sculptured superoccipital 
( s.o .), half of which is occupied by the arch of the great anterior canal ( a.s.c .), which is. 
margined by the sinus fossa and canal ( s.c .). The prootic (pro.) is seen to be connected 
by its wall-piece to the four-cornered alisphenoid ( a.s .), a narrow synchondrosis uniting 
them; the latter has not lost its fenestra ( a.s.f .). The pterotic (jpt.o.) is seen in the an- 
terior aspect of the lateral cerebellar fossa. 
There is now a basisphenoid (b.s.) as massive as the basioccipital, and containing, 
between its two clinoid walls (a.cl., p.cl.), the elegant cup-shaped cavity for the “in- 
fundibulum,” pierced below for the internal carotids (i.c.). In front of this cup the 
basisphenoid forms a thick low wall, above which the optic nerves diverge. Beneath the 
spheno-occipital synchondrosis the thick skull-base is made doubly thick by the massive 
basitemporal ( b.t .). In front of this superaddition the basal part of the equally extra- 
neous rostrum ( r.b.s .) is capped on each side by the basipterygoid cartilaginous 
meniscus (a.jp.). 
The presphenoidal region (_p.s.) is still soft, and the basisphenoid and perpendicular 
ethmoid are a long distance apart below ; but along the thick upper edge of the pre- 
sphenoidal cartilage there are now two pairs of bony plates — the fibrous representatives 
of the orbito-sphenoids. 
The hinder pair evidently correspond to the ectosteal bars that appear behind the 
fenestra in the huge orbito-sphenoids of the Lizard ; but as there are no cartilaginous 
“ alae minores,” the ectosteal plate degenerates into a thickish “ parostosis,” and then 
grafts itself upon the indistinct orbito-sphenoidal lips of the presphenoid. The addi- 
tional pair are very common in Birds ; and I have mentioned the existence of two centres 
in the Emu in my former paper (p. 137), where they are described as part of the pre- 
