548 FRANCIS VILLY. 
that may suggest cellular structure is only used for the sake of distinction. 
To avoid confusion the mesoblastic tissues, except those immediately con- 
cerned, are for the most part coloured uniformly. The perilymph and skull- 
cavities are finely dotted ; whilst cartilage is denoted by coarser dots.' Fig. 
6 and Figs. 13—19 are formed by compounding a number of sections so as 
to represent a single ideal and impossible one. In Figs. 15 and 16 the 
columella is the only part to which this description applies. 
Fie. 1.—Transverse section through the auditory region of an embryo 
frog, just before the coalescence of the neural folds. The auditory nerve and 
epithelium are already recognisable, although invagination has not yet begun. 
Fig. 2.—Similar section of a slightly older embryo, with the neural groove 
closed. On the left hand side the section passes through the centre of the 
invagination and cuts the whole length of the auditory nerve. Invagination 
is more advanced at the dorsal edge than it is at the ventral. On the right 
the section passes through the anterior part of the auditory involution. 
Fie. 3.—Similar section of an older embryo. The auditory invagination is 
cut through the centre on the left, and is more advanced than it is in Fig. 2. 
The nerve is shown. 
Fie. 4.—Similar section of an older embryo shortly before hatching, i. e. 
about 4 mw. in length. Invagination is now completed, and the vesicle is 
pyriform. 
Fic. 5.—Slightly oblique transverse section of a tadpole of 8 mm. On the 
left the section passes through the centre of the auditory vesicle, and the 
recessus labyrinthi is shown. On the right the section passes through the 
anterior part of the vesicle, showing the auditory ganglion and the epithelium 
common to the two anterior ampulle separating from the rest. 
Fic. 6.—Diagrammatic horizontal section of a tadpole of § mm., to show 
the gill-clefts and arches. The fore- and mid-brain are cut, as well as the nose 
and kidney. ‘The five pairs of clefts and six pairs of arches are represented. 
The hyomandibular cleft does not reach the skin. 
Fic. 7.—Oblique transverse section of a tadpole of 12 mm., showing the 
beginnings of the septa which cut off the semicircular canals. On the left 
the recessus is cut, along with the outer and upper septa belonging to the 
anterior vertical and horizontal canals respectively. On the right the anterior 
part only of the recessus is cut, and the outer ridge of the septum of the 
anterior vertical canal, together with the lower part of the septum of the 
horizontal canal. (x 25.) 
Fic. 8.—Section through a somewhat older tadpole than the last. On the 
right the horizontal canal is cut, its septum being completed. On the left the 
folds forming the septum of the posterior vertical canal are shown; they have 
1 The endolymph and contents of the perilymphatic canals are left white. 
