DEVELOPMENT OF THE SKULL OF THE COMMON FOWL. 
761 
The basi-hyal (b.h.), although formed in the united edges of symmetrical laminae, is, 
even as a defined mass of thickened blastema, quite azygous ; it is tear-shaped, the 
narrow end lying between the lower ends of the cerato-hyals ; this basal part is better 
defined than the arches, and chondrifies first. 
Behind the second fast-closing cleft (2) there is now to be seen a pair of well-defined 
rods, nearly as large as the Meckelian cartilages, and as early in their chondrification ; 
these are the “ branchials ” (c.br.), answering to the first branchial arch of the 
Ichthyopsida. There is a less distinct upper piece, pointed and small ( e.br .), and 
a delicate streak of ill-defined blastema behind and between ( b.br .) ; this latter is formed 
into the long slender so-called uro-hyal, the counterpart of the “ basibranchial band.” 
Behind the branchial arch a chink still indicates the place of the third visceral cleft (3)*. 
At this early stage the arch of the tongue has begun to lie within the mandibular 
arch, just as the lessened first branchial arch comes to lie within the massive hyoid arch 
in the Osseous Fish. 
Already the third visceral arch of the Bird has outgrown the second — a state of things 
universal in the Class of Birds. 
Before leaving this stage, it may be noted, as instructively parallel with that which is 
persistent in Cartilaginous Fishes, that there is no exo-skeleton, no prsemaxillse, maxillae, 
dentaries, angulars, or the like ; the mouth and unenclosed nostrils are entirely below 
the head : these, and the rudimentary tympanic cavities, open freely into each other, 
and are all lined with dermal tissue, which is, as it were, tucked in between the visceral 
laminae from the outer surface of the head. 
Second Stage. — Head of Embryo from 4 to 5^ lines long : 5th to 1th Hays of Incubation. 
In the course of twenty-four hours great changes have taken place in the chick’s skull 
and face. In an embryo with a head 4 lines long (Plate LXXXI. fig. 3, 6 diam.), 
a vertical section shows the notochordal region to be shorter than that formed by the 
trabeculae ; and this anterior moiety of the cranio-facial axis has become much straighter, 
the head having now to a great extent recovered from the “ cranial flexure.” The “ in- 
vesting mass ” has become more solid ; and a distinct swelling on each side, posteriorly, 
shows how the occipital condyle ( o.c .) is formed — very reptilian, however, in shape, at first. 
On each side, a thin soft lamina of cartilage ( e.o .) is growing up behind the third cerebral 
vesicle ( c.v . 3) ; this is the commencement of the occipital plane. At the angle formed 
by the bending of the primordial cranium upon itself, a transverse crest of cartilage has 
grown rapidly upwards and backwards; this, the “posterior clinoid wall,” protects the 
pituitary body behind, and lies in the midst of a quantity of delicate stroma, which 
* Till lately I have partly followed Professor Owen’s nomenclature for the third poststomal arch of the 
Bird, calling its moieties “ thyro-hyal.” (See Brit. Assoc. “ Keport on Archetype,” 1847, p. 295, fig. 23, where 
the lower piece is identified as the “ hypobranchial” of the Fish, and the upper is called “ cerato-hranchial.”) 
I find, however, that the “ thyro-hyals” of the Batrachia and Mammalia are merely retral “ hypobranchial” 
processes ; and I cannot consider the lower piece in the Bird to be other than the “ cerato-hranchial.” 
5 i 
MDCCCLX1X. 
