DEVELOPMENT OF THE EAR IN THE COMMON PROG. 161 



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 bj' 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 larvae of about 4 mm- 

 in length. No great advance in complication is to be noted until 

 the sacculus is marked out and the semicircular canals begin to form. 

 This does not take place until the larva is a well-developed tadpole 

 of 11 mm. 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 acces- 

 sory structures that arise in connection with the ear at a later period. 



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 con- 

 sisting 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 

 If 



