DEVELOPMENT OF THE SKULL OF THE COMMON FOWL. 
769 
parts of the skull-building ; and, unlike what the easier problem of the lower types, as 
a rule, presents, we shall see the outer works growing upon and grafting themselves into 
the inner works, and afterwards both the skeletons becoming compacted together and 
growing into the simplest brain-casket in one part, and the most mobile prehensile 
organ in the other. 
This third stage is in some respects parallel with what is seen in certain Teleostean 
Fishes, in which there is a free development of the cartilaginous endoskeleton, which is 
but little ossified, whilst the secondary or splint-skeleton is very frail and delicate ; such 
skeletons are possessed by the Lump-fish ( Cyclopterus ), and by the Salmon. 
In the illustrations of this stage the figures are drawn and magnified from embryos 
with the head two-thirds of an inch long ; here several of the ectosteal patches have 
appeared within the perichondrium, and the splint-bones are all present. The primordial 
skull, when stripped of its infoldings and seen from below, presents the appearance 
shown in Plate LXXXII. fig. 1 (magnified 10 diam.); here the nasal labyrinth is not 
shown. 
The prenasal cartilage (p.n.) has increased in size, and has become straightened so as 
to lie nearly on the same line as the septum nasi ( s.n .) and ethmoid ( eth .) ; it has not 
yet reached the acme of its growth, but has become spatulate at its end, and now well 
represents its large counterpart in the Plagiostomous Fishes, namely their azygous 
snout-cartilage. The base of the ethmo-vomerine plate (s.n., eth.) gradually narrows to 
near the pituitary space ( p.t.s .), where it rapidly enlarges, and has in its periphery first 
a pair of free plates ( a .p.), and then a pair of continuous lobes or outgrowths (l.g.). 
These free plates have, to me, only received an explanation after many years of work ; 
on referring to my former paper (Plate vii. fig. 4, a.p.), it will be seen that in an embryo 
of Struthio camelus, the counterpart in development with this stage of the chick, 
the “ anterior pterygoid processes ” (“ basipterygoids ” of Huxley) grow directly out 
of the roots of the trabeculae, in front and on each side of the pituitary space : the same 
thing takes place in the other Struthionidse (op. cit. Plate ix.-xv.) ; and this state of 
things is also seen in the Lacertilia generally, and exceptionally in the Mammalia (for 
instance, Cavia aperea*). 
Collateral work has prepared me for seeing into the metamorphic modification of the 
skull-base of the Fowl as compared with that of the Ostrich. If the reader refer to the 
memoir on the “Shoulder-girdle and Sternum” (plates 16 & 17), he will see a 
precisely similar modification of parts that form the shoulder-arch ; for, whilst the pre- 
coracoid of the Ostrich (plate 17. figs. 5 & 6, p.cr.) is continuous with the rest of 
* Here let it be remarked that my description of the base of the skull will differ considerably from that 
given in the Ostrich-paper. I have now found that the “ rostrum ” is not preformed in cartilage, that the 
pituitary floor is never filled in by cartilage, that the basitemporals are at first splint-bones and afterwards 
ectosteal plates, that, therefore, they do not perfectly correspond with the symmetrical basisphenoids of the 
Lizard, and, lastly, that, notwithstanding the different modes in which they receive ossific matter, the “ basi- 
pterygoid ” processes of the Lizard, the Ostrich, and the Cavy are all true representatives of each other. 
MDCCCLXIX. 5 K 
