772 
ME. W. K. PAEKEE ON THE STEUCTIJEE AND 
to the vagus foramen ; the bony matter is deposited on the inner face of the perichon- 
drium, and soon sets up endostosis in the subjacent cells. Although commencing on the 
outside, the deposit soon spreads round the selvedge, along the inner face of the cartilage 
(Plate LXXX1I. fig. 3, e.o.). The superoccipital lamina (Plate LXXXII. figs. 1 & 2, s.o .) 
is notched both above and below at the mid line ; it is soft at present, but in a day or so 
acquires a small external bony patch on each side of the mid line : this is a mammalian 
character ; and I know of only one genus (e. g. Turdus ) in which the superoccipital is 
azygous. The symmetrical bony deposits seen in fig. 1 ( b.s .) are borrowed , and will be 
described with the splint bones; the palatines (Plate LXXXII. figs. 1 & 7, pa.) are 
thoroughly ossified, and their subsequent increase is, in the Fowl, entirely at the expense 
of the fibrous investment, their primordial cartilaginous rod being the most transitory 
of any in the body ; their ossification is by endostosis, the deposit commencing in the 
central pith of oval cells. As far as I know, this is the only instance of “ primary endos- 
tosis ” in the skull of the chick ; in the old bird I shall describe a few endosteal patches 
in the nasal labyrinth *. 
But the pterygoid, which is best studied at this stage, is ossified, like an ordinary 
shaft-bone, by “ primary ectostosis ” (Plate LXXXII. figs. 2 & 2b, p.g.), although but 
little of the cartilage ever becomes hyaline. The ectosteal deposit (fig. 10, ect .) soon 
spreads amongst and ossifies the cartilage-cells, which are large, extremely thin-walled, 
fusiform externally, and passing into almost polyhedral cells towards the interior : they 
contain a large granular nucleus. These simple cells are rapidly converted into bone, 
but much later than in the palatine bar. An ectosteal patch has appeared on the 
broadest part of the quadrate (fig. 2, q.), and an ectosteal ring on the lower branchial 
(Plate LXXXI. fig. 15, c.br.) : these are all the bony deposits to be seen in the endo- 
skeletal parts of the head at this stage. 
The stapes (Plate LXXXI. fig. 14f, st.) has advanced very considerably in develop- 
ment, although entirely unossified; there can now be seen: — the wedge-shaped shaft (st.), 
the “ extrastapedial” process ( e.st .), which turns forward to apply its outer edge to the 
membrana tympani; the long antero-inferior process, the “ infra-stapedial” (i.st.), which 
grows forward to apply itself to the lower lip of the tympanic cavity, and then becomes 
related to the basitemporal ; and the crested postero-superior process, the “ suprastape- 
dial” ( s.st .), to which is attached an ascending ligament. In fig. 14 the relation of the 
stapes to the fenestra vestibuli (fo.) is shown; and in the section the squamosal ( sq .) is 
seen outlying the periotic wall ; then there is an open space above, which in the embryo 
is filled with an evanescent gelatinous tissue ; mesiad of this is a section of the anterior 
* The additional ossifications which appear in the palatine bar in other birds commence, in the patches that 
are developed into hyaline cartilage, as endostosis — as the “ transpalatines ” of the Passerinaj, and the azygous 
“ mesopterygoid ” of Ficus viridis. In the Eowl there is scarcely any primary endostosis in the body — merely 
in the carpals, tarsals, and epicnemial ; for, as in the “ Struthionidse,” their sternum is exceptionally ossified at 
first by ectosteal patches ( £ Shoulder-girdle and Sternum,’ plates 16 & 17, pp. 182-191). 
f The terms here used are taken from Professor Huxley’s paper “ On the Eepresentatives of the Malleus and 
the Incus of the Mammalia in the other Vcrtcbrata” Proc. Zool. Soc. 1869, pp. 391-407. 
