584 Proceedings of the Royal Society of Edinburgh. [Sess. 
II. 
My investigations were carried out upon a series of horse embryos 
which Professor Ewart kindly allowed me to study. The series comprised 
individuals from three weeks until twelve weeks, at weekly intervals, and 
then various stages up to full term. Of these one, aged four weeks, length 
12 mm., was already sectioned and mounted. Models were made of the 
auditory region of the five- weeks embryo and of a seven-weeks embryo. 
The three-weeks embryo was found to be so young, showing no skeletal 
development other than the chorda, as to be of no concern in the present 
discussion. 
The four- weeks embryo is important as showing the primitive condition 
of the visceral clefts. Fig. 6 shows a drawing of a section of the head of 
this individual passing through the region of the first and second visceral 
clefts. The first pharyngeal pouch (I. P.P.) is seen to extend latterly and 
to almost open into the first gill furrow, always, however, with a separating 
membrane between. From this point the first pharyngeal pouch is seen to 
be re-bent and to extend medially and dor sally. The second pharyngeal 
pouch (II. P.P.) extends caudo-laterally until it almost meets the second gill 
furrow (not shown in figure), being also separated from the latter by a 
membrane. The ideal condition of the pharyngeal pouches I have shown 
in a diagram, substantially the same as that employed by Fuchs (text 
fig. 1, A). The conditions met with in this embryo are different only in that 
there is an extension of the first pharyngeal pouch medially (marked with 
an asterisk in figures). This re-bent portion plays an important part in later 
topographical relationships, for the stapes is developed medial and dorsal to 
it. The mesenchyme is arranged in dense masses both cranial and caudal 
to the first pharyngeal pouch, representing the first two visceral arches , and 
cranial nerves are found running through the mesenchyme. 
Fuchs has contended that the mandibular arch is lateral to the first 
pharyngeal pouch. This statement is indefinite, because the first pharyngeal 
pouch has become extended in one portion while retaining primitive rela- 
tionships in others. In the ideal condition it is enough to state that a 
structure lies cranial or caudal to the first pharyngeal pouch to immediately 
define its position with regard to the visceral arches. But in the four-weeks 
stage there has already arisen a new complication, and we must give the 
relative position of a structure with regard not only to the pharyngeal 
pouch, but also to the prominent flexure of the pouch. Thus it will be seen 
that in the present stage the mandibular arch lies lateral to the re-bent 
portion of the first pharyngeal pouch, and also cranial to the point of 
