DEVELOPMENT OE THE SKULL OE THE COMMON EOWL. 
757 
whilst the whole of the notochordal region of the skull belongs to the occiput, this 
region, if it had suffered fission, would have sufficed to form a whole series of vertebrae. 
Thus that cranial region which has most of the vertebral nature in it cannot answer to 
one actual vertebral segment, seeing that it contains (subjected to the law of a new type 
of growth) several potential segments. 
As the skull proper cannot he shown to conform to the vertebral column, so neither 
can the facial arches be put into the same category with the ribs ; for the present they 
must wait until their development has been worked out perfectly in a considerable 
number of Vertebrate types : I mention this to excuse myself from all controversy con- 
cerning them in the present communication. The mouth and the fauces (the faucial 
region is of great extent in the Cyclostomous Fishes) are hedged in by what are 
evidently one class of cartilages ; these may be either free rods or fenestrate plates : I 
propose to call these the “ facio-faucial ” skeleton. 
These bars extend beyond the skull, and are very variably attached to it — in some of 
the lowest Vertebrates (as the Cyclostomi, the Chimseroids, and the Lepidosiren) being, 
anteriorly, mere outgrowths of the skull, whilst in the higher Classes they have a very 
independent development. In the Bird there are only three poststomal arches developed, 
whilst in the gill-bearing “ Ichthyopsida ” there are several. 
It seemed to me to be necessary to preface the description that is to follow with these 
remarks ; but I must now let the Bird’s head tell its own tale. What there may be in this 
account which will not harmonize with the descriptions given in the former paper will be 
duly noted from time to time, and the reasons for a modified nomenclature will be given. 
Structure of the Chick's Skull , First Stage. — Head of Embryo 3 lines long : 
4 th Hay of Incubation. 
The appearance of the chick’s head and face on the 4th day, as seen from below, is 
well shown in Professor Huxley’s figure (op. cit. p. 139, fig. 57 F) ; another figure (F) 
shows the cartilaginous cranium of the same. In Plate LXXXT. figs. 1 & 2, 1 have given 
what appears to me to be the state of things at this early stage ; fig. 1, however, the lower 
view, is somewhat diagrammatic, as I have in it anticipated the differentiation of the 
pterygo-palatine bar [jy.g.jya.) from the next stage ; these show the parts as magnified 
9 diameters. 
The visceral clefts (Plate LXXXI. fig. 1, 1,2,3) are still evident; and the fronto-nasal 
process ( f.n .) is, as yet, free at its sides, so that the anterior nares (n.) are still mere 
clefts — the foremost or prestomal clefts. The great mouth-cleft (m.) is four-sided, with 
concave sides and produced angles ; and the anterior angles run into the nasal clefts : 
these clefts, and their intervening arches, all evidently belong to one category; the 
arches surround, and the clefts open into the oro-faucial cavity. 
To show the character of the skull-floor, a chick’s head at this stage has been sliced 
completely through, horizontally (fig. 2), the second and third cerebral vesicles have been 
completely removed, and the first vesicle, the subdividing rudiment of the hemispheres, 
has had its upper half removed ; the lower half of each eyeball is shown also in situ. 
