1908-9.] Development of Auditory Ossicles in the Horse. 595 
has shown diagrammatically, that the stapes is cut off from a protuberance 
of the capsule, but in the model this slight swelling is seen to form the 
caudal boundary of the fenestra ovalis and to continue beyond the point of 
contact of the stapes. Moreover in the four- weeks embryo the vena capitis 
lateralis lies between the external surface of the head and the auditory 
capsule, and at this stage the auditory capsule shows no indication of any 
mesenchymic differentiation. In the five- weeks embryo the auditory 
capsule is quite distinct from the stapes, except for a very limited area. 
And the stapes at this stage is seen to lie not medial to the vena capitis 
lateralis, but dorsal to it. 
It will be recalled that in the four- weeks embryo the auditory capsule 
anlage lay between the seventh and ninth nerves. As development has 
proceeded, it is evident that the seventh and fifth nerves have come to lie 
more closely together, with a resultant shifting of the position of the 
capsule. But, as has been shown, the stapes arises in the area between the 
seventh and fifth nerves. 
I regret my inability to examine additional stages between those 
represented by the two embryos whose ages are four weeks and five 
weeks, but it is highly improbable that there would have been any 
great relative changes in the position of the various anlagen. To derive 
the stapes from the second arch would involve carrying it around the first 
visceral pouch. To derive it from the auditory capsule would imply 
a complicated process which would have certainly resulted in some more 
definite indication than we have in my second (five- weeks) stage. It is not 
at all surpising that the mesenchyme seemingly connects the capsule with 
the stapes, inasmuch as whenever two structures are developed closely 
together, the mesenchyme of their anlagen tends to become very difficult of 
visual separation. Moreover, if the auditory capsule and the stapes were 
from the same anlage, the stages of their development towards cartilage and 
later bone ought to bear a certain amount of relation one to the other. But 
this is not borne out by the facts. 
I have therefore come to the conclusion that the stapes cannot be 
developed from the auditory capsule. But if it is developed from neither 
the auditory capsule nor the second visceral arch, the question arises — 
Whence does it come ? 
The fact that the stapes lies medial and cranial to the first pharyngeal 
pouch, while it excludes the second arch, does not exclude the first arch 
from the formation of the stapes. For the stapes lies also dorsal to the 
first pharyngeal pouch, and cranial to the point of flexure. 
Fig. 1 shows also that the incus, which is undoubtedly derived from the 
