1908-9.] Development of Auditory Ossicles in the Horse. 583 
capsule itself, which hence makes that theory untenable. Fuchs would go 
further, and, since he conceived it to be plainly established that the 
mammalian stapes could have no relationship with the hyoid arch, derive 
the mammalian stapes also from the auditory capsule, and thus a homologue 
of the amphibian operculum and stylus. 
Our problem is, then, to determine the process of development of the 
mammalian sound-conducting apparatus, and to compare the relationships 
of its various components with the relationships existing between those ' 
parts having a similar function or a similar position in the lower vertebrates, 
in order to determine, if possible, the exact homologies. 
Before, however, discussing the actual development it is fitting to 
define an exact position with regard to one or two points. Fuchs has 
rightly emphasised Reichert’s distinction between the terms visceral arch 
(Visceralbogen) and visceral bar (Yisceralstreifen). A visceral arch consists 
of an axis of mesenchyme with an epithelial covering, which is endodermal 
on the inner side and ectodermal on the outer. Each visceral arch lies 
between a pair of visceral clefts, and in the arch is developed the visceral 
bar. Each visceral arch may give rise to one or more visceral cartilages, 
i.e. in general the first arch gives rise to Meckel’s cartilage and the 
quadrate ; the second arch gives rise to the hyoid or Reichert’s cartilage 
(often composed of various elements) and the hyomandibula. I have used 
the term “ visceral bar ” to refer only to Meckel’s or the mandibular cartilage, 
Reichert’s or the hyoid ean cartilage and the remaining branchial cartilages. 
Thus it will be seen that there is a distinction between the quadrate and 
the first visceral bar and between the hyomandibula and the second visceral 
bar. Further, it is clear that the visceral arch is an embrvonic structure in 
which certain skeletal elements later develop, while in the lower vertebrates 
the visceral bar is a permanent skeletal element. I shall try to show that, 
contrary to Salensky and some other authors, the only part of the ossicular 
auditory apparatus to have any connection with a visceral bar is the malleus. 
The term visceral cleft comprises two distinct kinds of structures, one 
invaginated from the exterior, the other from the pharynx. The former 
will be known as gill furrows, the latter as pharyngeal pouches. The first 
pharyngeal pouch persists as the Eustachian tube, and the first gill furrow 
as the external auditory meatus.* 
* It lias again been recently asserted that the Eustachian tube is not the lirst visceral 
cleft but the second (M. T. Sudler, “ The Development of the Nose and the Pharynx and its 
Derivation in Man f Amer. Jour, of Anat., i. 1901-1902). However, as the designation of 
the Eustachian tube as first or second pharyngeal pouch does not affect its actual relations 
as a structure lying between and separating the Meckelian and Reichertian cartilages, a de- 
tailed examination of that point is foreign to the present discussion. 
