DEVELOPMENT OE THE SKULL OE THE COMMON EOWL. 
785 
more generalized relatives on one hand, and from its higher and more specialized con- 
geners on the other. 
The period of a week has sufficed for very great changes in chondrous and fibrous 
tracts ; and now is the best nick of time for catching the true form of many of the osseous 
territories, although some have already lost their distinctness, whilst others have not yet 
appeared. The ectosteal tracts have set up “ endostosis ” in the cartilage lying between 
their plates, so that the true endoskeletal bones are being enlarged intrinsically as well 
as from the immediate fibrous layers. Save in the instance of the early grafted para- 
sphenoids, the parosteal tracts (splint bones) are still altogether free from union with the 
endoskeleton, whether bony or cartilaginous. 
The hyaline cartilage furnishing the interspaces and headlands of the ectosteal plates 
has become very dense through the abundance of the cheese-like intercellular substance; 
it is semitransparent when thick, and, for the most part, of a lilac colour. 
The splint bones are still fibrous, but are beginning to become smooth through the 
fresh and fresh ossification of aponeurotic layers; as a correlate of this exogenous growth, 
the first deposit of bony matter is beginning to be absorbed in many directions, so as to 
form diploe : this is most advanced in the basitemporals and squamosals. 
The drawings given of the ripe skull might, with very little modification, serve as dia- 
grams to illustrate the structure of the skull in any bird above the Struthionidse ; with 
the “ Praecoces ” they answer as to period ; but they correspond to nestlings of the 
“ Altrices” at about the end of the first week after hatching. 
The increased development of the occipital region is shown in Plate LXXXIV. figs. 
G-10. The basioccipital (figs. 6 & 10, b.o.) has nearly obliterated the notochord, and the 
bony substance is spreading both laterally and backwards into the cartilaginous condyle ; 
anteriorly (fig. 10) it has obliterated the spheno-occipital synchondrosis at the mid line. 
The form of the basioccipital is reptilian (lozenge-shaped) above (fig. 10), and mammalian 
(almost oblong) below (fig. 6). 
The exoccipitals (e.o.) are growing into large wings, which reach up to the mastoid 
(epiotic) region externally and above, and approach the superoccipitals at the foramen 
magnum (fig. 9) ; inside (fig. 7) they are still quite distinct from the opisthotic {op.) ; and 
both outside (fig. 8) and inside (fig. 10) they have enclosed the anterior and posterior condy- 
loid foramen (9) and the vagus-foramen (8). On the outer occipital plane (fig. 9), the 
exoccipitals trespass extensively upon the mastoid portion of the periotic cartilage 
(fig. 9, Ji.sc., p.sc.) ; the true mastoid (epiotic) has not yet appeared. 
The superoccipitals (fig. 4, s.o .) are fused together in their lower two-thirds, and form 
now a large shield of bone; internally (fig. 7) they enclose the whole crown of the huge 
anterior semicircular canal ( a.s.c .) and the sinus canal (s.c.), which burrows along the 
outer face of the arch and escapes near the inferior angle (figs. 7 & 9). 
The three periotic ectosteal plates which have at present made their appearance are 
all inside the cranial cavity (fig. 7). These are the front (prootic,^>ro.), the hinder (opis- 
thotic, op.), and the small representative of the great otic wing of the osseous Fish 
mdccclxix. 5 M 
