526 FRANCIS VILLY. 
The Invaginated Vesicle.—In tadpoles of this size, i. e. 
up to about 11 mm., the auditory sac is spherical in general 
shape, its wall consisting of a single layer of cells. At one 
point in the centre of the dorsal surface the regular contour is 
broken by a fold of the wall projecting slightly upwards, so 
that, as already mentioned, the vesicle is pyriform in outline (see 
fig. 4). This fold of the upper surface is at first only slightly 
marked off from the rest of the vesicle, but gradually it becomes 
narrower, and at the same time is pushed towards the inner 
side by irregular growth of the wall so that it comes to lie in 
the position shown in fig. 5. This fold is the first rudiment of 
the recessus labyrinthi. As mentioned above, it can be recog- 
nised from the moment the wall of the sac is completed, and it 
results from the mode of involution, but it is not a stalk con- 
necting the vesicle with the exterior at any stage. The typical 
Vertebrate ear retains its opening to the exterior for some 
time by means of such a recesssus labyrinthi, and the duct 
may even be present in the adult putting the endolymph in 
communication with the surrounding medium (Elasmo- 
branchs) ; but where the ear-fold concerns the inner epiblast 
alone no such communication takes place at all. In the 
case of the frog we should expect the recessus to be the last 
part of the vesicle to retain its connection with the external 
skin, but this is not so. This irregularity may perhaps be 
explained by the double-layered condition of the epiblast, 
which condition is most probably a secondarily acquired one. 
The epithelium forming the vesicle is more or less columnar 
at all points. The older part, i.e. that forming the wall next 
the brain, the floor and the recessus, consists of cells differing 
from the remainder in being more elongated. This is most 
marked at the lower inner angle where the cells are deeply 
pigmented. This tract of elongated cells extends upwards and 
outwards at the anterior end, especially in the later stages of 
this period. 
The auditory nerve runs from the inner wall, with the 
greater part of which it is connected, and reaches the brain 
just behind the recessus, The part applied to the vesicle is 
