524 FRANOIS VILLY. 
for study, embryos and tadpoles may readily be obtained at any 
desired stage, from the egg to the adult. 
The investigations here recorded have been conducted almost 
entirely by means of transverse sections, longitudinal sections 
being examined when points of special difficulty were 
encountered. 
Before passing on to the immediate subject of this paper I 
wish to take this opportunity of thanking Professor Marshall 
for the help that he has so readily given me, without which 
my work would have been lengthened, and rendered much more 
laborious than it has proved to be. 
Early Stages.—The earliest stage at which the auditory 
involution can be readily recognised occurs soon after the 
closure of the neural groove in the fore part of the embryo. It 
will be seen from fig. 2 that the invagination, though shallow, 
is perfectly distinct, the epithelium lining it being somewhat 
sharply marked off from the surrounding epiblast by the 
columnar character of its constituent cells. The invagination 
only concerns the deeper or nervous layer of epiblast, and is 
placed in the longitudinal groove separating the lower part of 
the embryo, distended with yolk, from the neural surface, and a 
little above the level of the notochord. The edge of the pit is 
roughly circular, and as yet the involution is regular, being 
deepest in the centre, and shallowing in all directions towards 
the edge. 
Although I have taken this stage as the earliest in the 
development of the ear, the auditory epithelium may be recog- 
nised with more or less certainty before the neural groove closes 
in. In these very early stages the deeper layer of epiblast is 
somewhat thickened in the region where the ear is formed later, 
and the auditory nerve may already be traced into continuity 
with it. 
As development proceeds the involution is more rapid at the 
dorsal than at the ventral part, so that in section it comes to 
have the appearance shown in fig. 2. After this has taken 
place the ventral part in turn grows rapidly, pushing its way 
inwards (see fig. 3), and as this process is going on the lips of 
