DEVELOPMENT OF EAR AND AOOESSORY ORGANS IN FROG. 525 
the pit grow towards each other and coalesce, thus closing the 
aperture completely. Soon afterwards the newly formed outer 
wall of the vesicle separates from the deeper layer of epiblast, 
which remains near the surface. I am not certain that the 
mouth of the auditory sac is closed precisely in this manner, 
for the nervous layer of epiblast is so ill defined at this time and 
place that I have thought it more probable that the outer wall 
of the vesicle is formed simply by the cells at the edges of the 
pit proliferating, and closing the mouth by forming a plate 
across it. 
Whatever the real mode may be, the vesicle is pyriform 
when first closed, the dorsal part being the narrower. This 
narrow part ultimately becomes the recessus labyrinthi. The 
outer wall of the vesicle, being the latest formed, consists at 
first of cells indistinct in outline, but soon they assume, more 
or less completely, the columnar character of the opposite 
wall, though the two parts never agree exactly, as the older 
contains more pigment than the younger. It is from the old 
pigmented parts that the sensory tracts of the adult ear are 
formed. 
Very soon after the auditory sac is completely closed, irregular 
mesoblast cells make their way into the space between it and 
the external epiblast, and by this means the organ is removed 
farther and farther from the surface. 
The whole of the changes mentioned above take place before 
the tadpole character is assumed, and the invaginated vesicle, 
such as is here described, may be found in just hatched larvee 
of about 4 mm. in length. No great advance in complication 
is to be noted until the sacculus is marked out and the semi- 
circular canals begin to form. This does not take place until 
the larva is a well-developed tadpole of 11 or 12 mm. in 
length. 
I propose now to give a short description of the organ as 
present in tadpoles of this latter size, and then to describe the 
changes that take place in the various parts during the older 
stages, and the accessory structures that arise in connection 
with the ear at a later period, 
