14 
ME. W. K. PAEKER ON THE STRUCTURE AND 
the articular region, and a rudiment of the well-known avian “posterior angular 
process ; ” in Mammals the former is called the “ manubrium of the malleus.” Under 
the proximal part of Meckel’s cartilage, where it becomes the massive articular 
region, there is an ectosteal deposit—the well-known bony tract which develops into 
the articulare in oviparous types, often having an endosteal centre added to it; it is 
the “articulare externum ” (Plate 2, fig. 4). Here it forms the outer rudiment of the 
bony malleus ; it is partly hidden by the “ annulus” ( [a.ty.). 
In comparing the next or proximal segment of the first visceral arch with that of 
the Bird—the quadratum—there are two things to be noticed: first, that the “orbital 
process ” is suppressed; and secondly, the head of the segment is articulated with the 
cranium further forwards than in most Birds.* 
There is a neat head to the upper crus of this cartilage, called in human anatom v 
the “short crus of the incus ” ( s.c.i .), and this fits into a neat cup in the cartilage of 
the ear capsule, above, just between the ampullae of the anterior and horizontal canals 
( a.s.c.,k.s.c .). Below and behind the saddle-shaped condyle, the body of this segment 
narrows into the “ long crus” ( l.c.i.), and this part acquires an inturned “neck,” very 
slender, which carries a discoid head'—the pars orbiculare. 
This is another and new, or Mammalian, character; here the top of the first visceral 
arch keeps outside the top of the second, as in Fishes, generally, and amongst 
the Sauropsida in the Chameleon (see Trans. Zool. Soc., vol. xi., plate 16, figs. 4 
and 7); whereas in many Amphibia and Sauropsida the topmost hyoid element—the 
columella—rides over the quadrate. 
The great specialisation which these parts have undergone, which they show so 
early in the embryo, is seen in the tilted position of the semicircular canals ( a.s.c ., 
h.s.c., p.s.c.) ; for in relation to the axis of the skull, the horizontal canal is almost 
vertical, and the two vertical canals are quite oblique in position. Thus the hinder 
part of the “ tegmen tympani ” ( t.ty .) forms a recess for the stapes (st.) and its muscle 
( st.m.) ; this is quite like the recess formed by the paroccipital wing in the Bird’s 
skull. 
The form of the stapes (which will be shown in the next stage) is quite normal; 
it is a short, flat, perforated column, the oval base of which fits accurately into the 
fenestra ovalis. In this figure it lies in the shade, within the incus, and the stapedius 
muscle (st.m.) is seen arising, tendinous, from its neck, and enlarging, backwards, to 
be inserted, fleshy, in the bottom of the recess at the back of the tympanic cavity. 
As the parts in this early stage were newly, but completely, chondrified, I had a 
good chance for seeing the exact relations of the parts of the hyoid arch. 
The stapes, as the counterpart of the Skate’s hyomandibular, chondrifies separately ; 
it is the only pharyngobranchial element developed in the walls of the Mammalian 
* I lay stress upon further forwards; no one lias ever dreamed of putting the Bird’s quadrate into 
the hyoid category, notwithstanding that its articulation with the occipital arch is partly behind the 
auditory capsule. 
