588 Proceedings of the Royal Society of Edinburgh. [Sess. 
stapes anlage lies medial and dorsal to the first pharyngeal pouch. The 
stapes mesenchyme, moreover, extends to the crus longum incudis, and there 
is no point at which the incus and stapes may be said to be distinct * This 
relation is shown in fig. 10. The stapes anlage at one point lies very close 
to the auditory capsule, so close, in fact, as to be almost indistinguishable 
from it, but always we may find a line of demarcation, whereas the connec- 
tion between stapes and incus is exceedingly close, the mesenchyme having 
at all points an equal density. Moreover, the crus longum and the stapes 
show no signs of differentiation into cartilage. 
The hyoid bar as cartilage or pro-cartilage extends somewhat farther 
dorsal than the mandibular cartilage. It then contracts and sends a more 
or less broad band of mesenchyme to meet the stapes anlage. This is the 
hyostapedial ligament of Huxley and others, shown in the figures marked 
with a double asterisk. It is important to note that the entire auditory 
ossicular apparatus is at this time represented by a more or less homo- 
geneous mass of mesenchyme. It is impossible to distinguish sharply 
between the proximal ends of the hyoid cartilage, Meckel’s cartilage, the 
malleus, incus and stapes. Moreover, it is extremely difficult to draw a line 
sharply between the stapes and the auditory capsule — although apparently 
the stapes is much less intimately connected with the latter than with the 
other elements. 
I have spoken of the mesenchyme anlagen of the auditory ossicles as 
being more or less homogeneous. There are, however, within the area 
certain differentiations which it is important to notice. 
Fig. 8 (M) shows the Meckelian cartilage beginning to elongate cranio- 
caudally. This is the first indication of the malleus. The mandibular bar 
is carried up and into the malleus by a region of advanced pro-cartilage. 
The malleus is scarcely yet in the condition of pro-cartilage, but its outlines 
are fairly easy to distinguish by a somewhat denser arrangement of the 
mesenchyme. Proximal to the malleus, indications of pro-cartilage are 
observed to occupy a somewhat different position. Fig. 9 is a higher power 
view of a section distal to fig. 8, and shows the indication of an articulation 
between the malleus and incus. It will be seen, then, that pro-cartilage is 
found in the articular portion of the incus and in the distal end of the 
malleus ; that pro-cartilage indications are found in the malleus, and pro- 
cartilage is present in the connecting rod between the malleus and the 
* I found in examining sections of rabbit of seventeen days (length 13*5 mm.) and a 
mole (length 10'5 mm.) which Dr Beard kindly allowed me to use, that this connection 
between stapes and incus was less marked in the mole than in the horse, and lacking entirely 
in the rabbit. However, both individuals (mole and rabbit) were further developed than 
the four-weeks horse. 
