1908-9.] Development of Auditory Ossicles in the Horse. 587 
from the first gill furrow by a space of considerable extent. We have here 
a condition similar to that found in the four-weeks embryo, except that 
the first pharyngeal pouch is thus separated from the first gill furrow. 
Moreover, the mesenchyme in extending down between the two structures 
might he conceived to have carried with it structures derived either from 
the mandibular or hyoidean arch. But it will be possible to determine from 
which arch these structures come, provided that they retain some close con- 
nection with their respective arches. If a structure lies both cranial to the 
flexure of the first pharyngeal pouch and lateral to the re-bent portion, that 
Fig. 1. — V. A., visceral arch ; G.F., gill furrow ; P.P., pharyngeal pouch ; 
S. , anlage of the stapes. 
structure is clearly in the region which is without question mandibular 
and which is, in fact, the same region which we have defined as the 
mandibular arch in the four- weeks embryo. It is in this position that 
the anlage of the incus lies, and about the ontogenetic relations of the 
incus there can be then no question. It is clearly developed from the 
first visceral arch. 
Medial to the incus anlage a spherical mass of mesenchyme arises, 
perforated by a minute branch of the carotis interna. This, the anlage of 
the stapes, lies medial and dorsal to the first pharyngeal pouch and cranial 
to the prominent point of flexure. F uchs states, in contradiction to Broman 
(who stated that the stapes arises caudal to the first visceral pouch), that 
the stapes lies medial to the first pharyngeal pouch, and with this view I 
have, in an earlier portion, concurred. But this is not entirely exact. The 
