DEVELOPMENT OP THE SKULL OF THE COMMON FOWL. 
763 
older. I shall also leave the mandibular ( m.k .) and hyoid cartilages (A.) for description 
from an older embryo. 
The development of the parts goes on very rapidly, so that when the head is 5 lines 
in length a much clearer idea of the structure of the parts can be obtained. A vertical 
section, left-hand of the septum, of such a skull is shown in Plate LXXXI. fig. 4, and 
a side view in fig. 5, both magnified 6 diameters; in these views the cerebral vesicles are 
not figured. 
A low ornithic type, such as is persistent in the Ostrich, is now attained. I have 
already had to compare the skull of the chick with that of a Plagiostome and of 
a Turtle ; after the Struthious stage has been passed, many of the higher ornithic spe- 
cializations will rapidly appear. 
The straightening of the cranio-facial axis has gone on so as to make the face much 
more prognathous ; and the axis of the anterior part of the face (jp.n.) is especially 
altered, so as to have already taken on an ornithic character ; the boundary, also, between 
the nasal and orbital regions can now be seen. One landmark has become especially 
visible, on account of the denser condition of the cartilage ; this is the bridge for the 
nasal nerve ( n.n .) : in the adult bird this is seen near the free postero-inferior angle of 
the nasal septum. For some distance behind this bridge the cartilage continues thicker 
than we find it between the eyeballs ; the larger, thinner, posterior part of the cranio- 
facial axis answers to the whole of the presphenoid and the posterior or interorbital 
part of the perpendicular ethmoid. The grooves for the olfactory nerve (1) can be well 
seen ; and the retral spike of the ethmoid surmounting the nerve-grooves has become 
much more defined, and has begun to take a backward direction instead of the vertical 
and human direction it had a day or two before (see Plate LXXXI. fig. 3, etli.). This is 
now the highest part of the cranio-facial axis, whereas the presphenoid was the highest 
in the last embryo. The whole base of the prepituitary part of the primordial skull is 
very thick and rounded ; and its thickness from side to side increases in the prenasal 
cartilage {p.n.), which now shows itself to be a very fit model on which the premaxil- 
laries of any type of Bird might be formed ; in no Bird whatever, let the shape of its 
face be what it will, is the type of the first model or platform wholly lost, although in 
every case the primordial structure (into the type of which they were cast, as it were,) 
undergoes very early wasting. Between the palatal skin below, and the still very arcuate 
skull-base above, a considerable mass of stroma is formed, which is cordiform in section, 
and which soon becomes the seat of the rostral (or anterior parasphenoidal) ossification : 
this rostral stroma ( r.st .) is shown in fig. 6, magnified 12 diameters. This tract is com- 
posed of thin-walled baggy cells, which never chondrify (except on the surface in certain 
birds, as the Psittacinse and Ardeinse), but develope either into bone (internally) or into 
periosteum (externally). In fig. 7 the differentiation of the tissues that underlie the 
skull-base in the posterior ethmoidal region is shown, magnified 160 diameters. The 
ethmoid ( eth .) is already formed of ordinary hyaline cartilage, with abundant intercel- 
lular substance and tear-shaped cells; next below this is a tract of very delicate granular 
5 i 2 
