1908-9.] Development of Auditory Ossicles in the Horse. 598 
cranial border of the first pharyngeal pouch, a minute branch is given oft" 
which runs cranio-laterally and perforates the stapes anlage, thence pro- 
ceeding cranio-laterally between the vena capitis lateralis and the anlage of 
the malleus and incus. Now, were the arteria stapedialis clearly the second 
aortic arch, one would be justified in considering that the perforation of the 
stapes by it was strong evidence in support of the claim that the stapes 
anlage were closely related to the second visceral arch. Tandler has shown 
that in the case of the rat and of man the arteria stapedialis is derived 
from three different structures. From the point at which it arises from the 
aorta dorsalis up to the stapes, it is the dorsal portion of the second aortic 
arch. From that point on the artery with its rami consists of structures 
which are either developed secondarily or from the first aortic arch. Thus 
the portion which actually perforates the stapes anlage is not developed 
from the second aortic arch. Moreover, the stapes of monotremes and of 
certain marsupials is not perforated by an artery. So then, if the stapes 
is developed similarly throughout the mammalia, the relations between 
it and the arteria stapedialis cannot be regarded as indications of its 
derivation. 
Thus it is seen that there is at present no evidence weighty enough to 
overbalance that drawn from considerations of relative position, and we 
must therefore conclude that the stapes is related neither to the second 
visceral bar nor to the second visceral arch. 
Fuchs arrived at this same conclusion, but went further and declared 
the stapes to be derived from the auditory capsule. We have seen that 
there is difficulty in visually separating the stapes from the auditory 
capsule at all points. Now let us agree that the stapes is developed from 
the mesenchyme of the capsule, and with that in mind examine some more 
fully developed stages. 
Text fig. 2 shows a section through the stapes and incus of an embryo 
of six weeks. The stapes and incus are just between a condition of pro- 
cartilage and true cartilage, and the cochlear portion of the auditory 
capsule is in the same developmental stage. The fenestra ovalis is clearly 
to be seen closed by a mesenchymous membranous indication. The vesti- 
bular portion of the capsule is still undifferentiated mesenchyme. The 
stapes is seen to be impinged against the closing membrane of the fenestra. 
Text fig. 3 shows a section through the same region of a seven-weeks 
embryo. Unfortunately the sections are not upon exactly the same plane, 
so that the one shows the incus, the other the vestibular portion of the 
capsule. However, the relations between fenestra ovalis and stapes are 
similarly shown. All the parts are now cartilaginous. The stapes is 
vol. xxix. 38 
