DEVELOPMENT OF THE SKULL OF THE COMMON FOWL. 
795 
pterotic (pt.o.) ; its sutures have also disappeared from the inner face of the skull. The 
suture between the prootic and exoccipital ( pro ., e.o.) is dying out at the middle, and on 
the inside only the upper part of the suture between the prootic and opisthotic remains ; 
there is just a trace also, inside, of the suture between the exoccipital and the opisthotic. 
The whole para-basisphenoidal mass has long been one bone ; the posterior wings (p.rp.) 
strongly clamp the infero-anterior edge of the prootic (pro.), both above (fig. 5) and below 
(fig. 6). There are a pair of anterior clinoid spurs (fig. 5, a.cl.) ; behind these a pair of 
lateral spurs; and the posterior clinoid chink (fig. 5, p.cl.) is filled up with a mass of 
periosteal bone ; behind this mass is the primordial chink, the remnant of the posterior 
fontanelle. The prepituitary part of the basisphenoid (figs. 5 & 6, bs.) has not nearly- 
reached the perpendicular ethmoid ; the latter part, with its surrounding cartilage, has 
been macerated away, leaving the rostrum (r.b.s.) as a long grooved style, having at its 
root the anterior pterygoid processes with their cartilaginous facets (a.p.). 
The alisphenoids were still connected with the basal mass by synchondrosis, so that 
these do not appear in the figures. 
The head of the quadrate at this stage is shown in fig. 8 ; the squamosal facet is a 
hemisphere deficient at the inner side ; there is then a ligamentous line, and below this, 
on the inside, a somewhat reniform subconvex facet for the subconcave facet on the fore 
end of the rod-shaped rib of the prootic (fig. 6, pro., q.c.). 
Ninth Stage. — Young Fowls of the ls£ Winter ; from 7 to 9 Months old. 
The next stage may be considered to cover a period of two or three months, the 
specimens examined being such as were hatched in spring and killed at Christmas ; here 
the approaches by which the adult condition is obtained are plainly shown. 
A sectional view at the earlier part of this stage (Plate LXXXVX. fig. 14) displays the 
growing massiveness of the skull and the gradual closure of the sutures. Two extensive 
regions of the skull, namely the posterior sphenoid and the occipito-otic, now form one 
mass of bone (fig. 14, s.o., e.o., b.o., b.t., b.s., r.b.s., pro., op.); but the larger (a.s.) and 
lesser (o.s.) wings of the sphenoid are still separate. The huge perpendicular ethmoid 
(fig. 15, eth. ; fig. 14, p.e.) now, more than ever, displays its special ornithic development. 
I have failed to trace any upper osseous centre such as appears in the aberrant Struthi- 
onidse ; but the right and left ectosteal plates, continuous in front, set up endostosis in 
the enclosed wall of cartilage, and spread upwards, downwards, and backwards. Here 
the triangular osseous patch which appears on the upper surface of the skull, between 
the diverging frontals (fig. 15, eth.,f), is not the same bone as that which shows itself 
in the same gap on the Ostrich’s head (First Paper, Plate vm. fig. 3, eth.) : that is an 
upper ethmoid; in the Fowl it is merely a continuation of the perpendicular plate. 
The bony substance spreads at the top laterally into the roots of the aliethmoids, 
forwards into the cranio-facial isthmus (between eth. and s.n. in fig. 14), and backwards 
into the retral process which overlies the olfactory grooves (1). Also below these 
grooves ossification is spreading into the ethmo-presphenoidal bar ; in this specimen it 
5 isr 2 
