1908-9.] Development of Auditory Ossicles in the Horse. 585 
flexure. Thus to define the mandibular arch region as being separated 
from the region of the hyoid arch by a line passing from the extreme 
distal end of the first pharyngeal pouch to the first gill furrow is obviously 
a mistake. The plane of separation in this case passes through the 
prominent point of flexure of the first pharyngeal pouch and through the 
first gill furrow. 
Although this embryo shows no skeletal elements which may be 
definitely identified as such, it is important to further note the condition of 
the future auditory capsule. 
In fig. 7 a section is shown through the auditory sac. This appears 
as a hollow subspherical mass of ectodermal tissue, lying in the mesenchyme 
between the seventh and ninth nerves. The fifth and eleventh nerves 
are seen cranially and caudally respectively. Around the auditory 
sac the mesenchyme is seen to he in a condensed condition, which is an 
indication of future cartilage formation. Thus we have here the anlage 
of the auditory capsule, in a very primitive condition. In view of the 
contention of Fuchs that the stapes is developed from the wall of the 
auditory capsule, the position of the capsule as it here appears is very 
important. 
The seventh nerve runs latero-caudally from the ganglion shown in 
fig. 7, and crosses the vena capitis lateralis (not shown in the figures) 
and comes to lie between the latter structure and the outer surface of the 
head. It then runs caudally and ventrally, coming to lie in the position 
shown in fig. 6. The vena capitis lateralis shows at this stage the same 
relations as in the later embryos, that is, it runs dorsal and lateral to the 
auditory region, being crossed by the seventh nerve, until it reaches the 
region of the ganglion of the fifth nerve, where it bifurcates and loses its 
importance in this discussion. 
Figs. 1, 2 and 3 show three drawings of a model of the auditory region 
of a five-weeks horse embryo, length 15‘5 mm. The skull shows no sign of 
cartilage formation except in the posterior cranial portion (chorda and 
trabeculae) and in the first two visceral bars. The cartilage in these latter 
structures extends as far dorsal as to the area to be described as the anlage 
of the malleus, and is shown coloured blue in fig. 8. It is here scarcely yet 
in the condition of true cartilage, but it is in a very advanced stage of pro- 
cartilage. More dorsal to the point figured in fig. 8, cartilage disappears 
entirely except for a small area in the anlagen of the malleus and incus, to 
be described later. It will thus be seen that the dorsal (proximal) portions 
of the first two visceral bars are less advanced than their ventral (distal) 
ends towards cartilage formation. Lateral and somewhat dorsal to the first 
