DEVELOPMENT OE THE SKULL OP THE COMMON FOWL. 
771 
which is a crest of cartilage altogether continuous with the investing mass, and the root 
of the corresponding trabecula : this part will be better shown in the description of the 
fourth stage. On each side, and a little behind the posterior fontanelle, the investing 
mass rises into an elegant mammillary swelling ; this is seen both above and below, and 
arises from the enclosed cochlea, the cavity of which, and the lower otoconial deposit, 
can be well seen when the skull is viewed by transmitted light (see Plate LXXXII. 
figs. 1 & 3, c.l.). These swellings reach to within a small distance of the notochord (n.c.) ; 
on the outside they are bounded, on the lower face of the skull, by the carotid groove 
( c -9-)-> an d posteriorly they nearly reach to the foramen for the vagus nerve (8). The noto- 
chord (n.c.), which lies above as well as between the moieties of the investing mass, has 
relatively receded, through the more rapid growth of the surrounding structures ; it is 
now at its fullest development, and osseous tissue has appeared on its upper surface, 
behind. Although belonging, in the early embryo, to the basioccipital region only, the 
notochord becomes constricted in several places (Plate LXXXII. fig. 3, n.c.) as if it were 
ready, if the investing mass were so disposed, to undergo hourglass-like segmentation. 
The cells of which the swollen parts are composed are oval, thin-walled, and contain 
many granules (Plate LXXXII. fig. 5, 250 diam.) ; they form a good instance of cellular 
or indifferent tissue, as the intercellular substance is almost inappreciable. The osseous 
deposit takes place at first on the posterior third, on the upper face of the rod (fig. 3), and 
it appears as a fenestrate ectosteal plate (fig. 6, n.c., 250 diam.): when once set up, the 
parent cells below soon disappear ; and after a while the bony substance spreads over 
the investing mass (i.v.) on each side, and sets up endostosis in that part which, soon 
acquiring an inferior ectosteal place by the extension of ossification through the pri- 
mordial chink, lays the foundation of the triple basioccipital bone. 
Posteriorly another pair of smaller, but more convex mammillse, are formed 
(Plate LXXXII. fig. 1, o.c.), these are the occipital condyles ; between these, which have 
coalesced largely below, and in some degree above (fig. 3, o.c., n.c.), a portion of the 
notochord projects, part of which belongs to the “ atlas.” 
Outside the carotid groove the periotic portion of the investing mass becomes scooped 
into an oblong rounded sulcus. This sulcus has only a low wall in front, but is bounded 
behind by a large ear-like flap of the exoccipital lamina ; this latter is the “ tympanic wing 
of the exoccipital:” and the sulcus is the rudiment of the tympanic cavity (Plate LXXXII. 
%• 1? ty) ; at present it is filled with a soft flocculent stroma, soon to be absorbed. 
The upper head of the quadrate is loosing its connexion with the occipital ala at the 
end of the sulcus, and is acquiring a cup-and-ball articulation with the fore part. 
The low ridge bounding the sulcus externally is the rudiment of the “ tegmen tympani” 
of the Mammal ; the lower lip of the tympanic cavity is supplied by the fibrous skeleton, 
there being no such cartilaginous lamina as is seen in the highest Class. 
At present the whole of the periotic capsule is unossified ; but, besides the notochordal 
shaft, the occipital region has acquired a pair of triangular bony patches ; these are the 
exoccipitals (Plate LXXXII. figs. 1 & 2, e.o.) : they extend from the posterior selvedge 
5 k 2 
