DEVELOPMENT OE THE SKULL OF THE COMMON FOWL. 
777 
An upper view of the primordial skull at this period shows much that is noteworthy (see 
Plate LXXXIII. fig. 2, magnified 4 diam., and with the basal parts left out of focus). The 
nasal labyrinth is relatively very large, and posteriorly overhangs the orbital region ; it is 
at once seen how large and important the structures are that are trabecular in their origin ; 
their territory ranges from the pituitary space ( p.t.s .) behind, to the end of the prenasal 
cartilage (p.n.) in front ; the postpituitary or notochordal region is short, but very broad. 
In this upper view of the skull-base the basioccipital ( b.o .) is seen to have the form 
of a flint spear-head : it has a longitudinal ridge arising from the enclosed notochordal 
shaft ; its point lies between the divisions of the occipital condyle (o.c.), and its blunt 
end lies between the cochlear elevations ( c.l .). The cartilage does not extend far 
beyond the cochleae, antero-mesially, but has retired far from the mid line ; this state of 
things, combined with the rapid growth of the investing mass and the arrest of the 
notochord, has caused the “ posterior basicranial fontanelle ” (p.bf) to become a large 
lozenge-shaped space, merely filled with delicate stroma. 
The exoccipitals ( e.o .) are spreading ; and the superoccipitals (s.o.) are large, and are 
approaching the mid line. 
The prootics (pro.) are rapidly developing as ectosteal plates, in front of and below 
the ampulla of the anterior semicircular canal (a.s.c.) ; and a large ectosteal tract covers 
the middle part of the alisphenoidal region. The pituitary space ( p.t.s .) is enclosed with 
derived bony matter ; but a considerable amount of the anterior (a.cl.) and posterior 
(p.cl.) clinoid walls are still soft. All the rest of the skull (its trabecular portion) is still 
soft, and the notochordal regions are also, to a large extent, cartilaginous. The tem- 
poral wings of the exoccipital and the postfrontal outgrowths of the alisphenoid 
(pf., a.s .) are seen to be large masses of cartilage. The foramina ovalia (5) are very 
large fenestral spaces between the alisphenoidal and periotic regions ; and the foramina 
rotunda (5 a.) are oval in shape, and pierce the root of the “ greater wing.” The centre 
of the alisphenoid is occupied by a fenestrate cleft (a.s.f.) ; this is cordate, with the apex 
downwards, and tends to divide the ala into two bars — an antero-mesial and a postero- 
external. The alisphenoids are wide apart mesially (exaggerated in the figure), and 
there is no more cartilage until we reach the mid line, occupied by the knife-shaped pre- 
sphenoid (p.s.). Thus not only is the roof of the primordial skull occupied by an 
enormous fontanelle, but there are two mesial fontanelles in the lowest part of the floor, 
and its anterior or upper division is almost entirely membranous. 
The division between the two basal fontanelles (the posterior clinoid wall) is imperfect 
above; in my former paper (Plate vii. fig. 1, & Plate ix. fig. 1, p.cl.) this is shown to be 
perfect in the Struthionidse. In the embryo of Struthio camelus (Plate vii. fig. 1, o.s.) 
there is also a considerable tract of cartilage underlying the fore part of the hemispheres ; 
this is formed of the lesser wings (orbito-sphenoids) : these are aborted in the Fowl*. 
* The development of the sphenoidal region of the Fowl (and of Carinate birds generally) is thus seen to he 
clean contrary to what obtains in the Lacertilia, where the alisphenoids are absent, and the orbito-sphenoids are 
very large, fenestrate, and have (like the alisphenoid of the Bird) several osseous centres. 
MDCCCLXIX. 5 L 
