760 
MR. W. K. PARKER OK THE STRUCTURE AND 
the pith of this bar, my study of which has been from fine transverse sections in the 
next stage, is somewhat anticipated. The axis of this soft bar is converted into the 
palatal and pterygoid rods (pa.,p.g.); and from the outer edge of each of these bent 
clubs a lamina grows downwards (see Huxley, op. cit. fig. 57 G, l.), in the internal 
fibrous stroma of which the maxillary and jugal bones are afterwards formed. The 
axis of the maxillary rudiment never has any cartilaginous connexion, anteriorly, in the 
Bird, with the ethmoidal lamina (prefrontal) ; and, behind, the chondrification of the 
pterygoid portion of this axis is quite distinct from, and much later than, that of the 
“ quadrate.” Here we see that the “ subocular arch ” is much modified from what is 
seen in the Ichthyopsida. 
The primordial skull figured by Professor Huxley (fig. 57 F) must have been from a 
less mature embryo than that which I am here illustrating as my first stage ; for in that 
figure the quadrate (qu.) is shown as continuous with the antero-external part of the 
“ investing mass.” In my specimen (Plate LXXXI. figs. 1 & 2, qu.) the quadrate is already 
a very distinct tuberous mass of consistent cartilage, rapidly becoming hyaline. Meckel’s 
cartilage (m.Jc.) is also very distinct and solid, and its articular end has a sinuous face, 
answering to that on the lower end of the quadrate ; whilst the posterior and internal 
angular outgrowths have already appeared (fig. 1, m.Jc.). These Meckelian rods are very 
short in proportion to their thickness, are gently curved forwards, and nearly meet at 
the mid line by rounded ends. These axial parts of the first pair of poststomal laminee 
are surrounded by very delicate blastema — the parent tissue of the splints, muscles, and 
skin of the lower jaw ; the outline of this primordial mandible is emarginate at the mid 
line. That which is perhaps the most noteworthy in the first poststomal arch at this 
stage is its position with regard to the auditory capsule ; its pier, the quadrate, is seg- 
mented from the antero-external angle of the investing mass, directly in front of the 
periotic capsule, and opposite the junction of the anterior with the middle third of the 
cranial part of the notochord ; whilst the fifth nerve (5) passes out in front of it. This 
normal position is lost and then regained before the chick is ripe. Close behind the 
quadrate there is the first poststomal cleft (1), ready to become converted into the 
complex tympanic cavity. 
In the obtuse angle formed by the Meckelian rods, and lying on a higher plane than 
those rods, we find the distal arrested cartilages that belong to the second poststomal 
arch; these are the “ cerato-” and “basi-hyals” (Plate LXXXI. fig. 1, c.h., b.li.). At 
present these are merely thickened masses of blastema, their outline being somewhat 
indistinct, as the young cells of which they are composed are not sharply separated from 
the formative substance which will become their perichondrial investment. The cerato- 
hyals (c.h.) are even now only half the size of the mandibular rods ; but this is the 
greatest relative size they attain, for they diminish much afterwards. 
There is no proper skeletal substance between the top of the cerato-hyals and the 
tissue that forms the auditory “ columella” (stapes) : this latter part is not differentiated 
sufficiently to be described at present ; it is both late in appearance, and also very minute. 
