DEVELOPMENT OF THE EAR IN THE COMMON FROG. 181 



Ampulla of the posterior vertical canal. P. V. C. Posterior vertical 

 canal. Q. Quadrate cartilage. B. Recessus labyrinthi. S. Scapula. 

 Sac. Sacculus. S. A. Septum of anterior vertical canal, outer fold. 

 S.H. 1. Septum of horizontal canal, upper fold. S. H. 2. Septum 

 of horizontal canal, lower fold. S. P. 1 . Septum of posterior vertical 

 canal, outer fold. S. P. 2. Septum of posterior vertical canal, inner 

 fold. St. Stapes. Stn. Sternum. Su. Suprastapedial cartilage. 

 Su. s. Suprascapula. T. Tail-muscles. Th. Thymus. Ty. Tympanic 

 cavity. U. Utriculus. V. Vagus, in. Third nerve. 



These figures are all more or less diagrammatic in different ways, 

 but in all the general outlines are as nearly correct as possible, for 

 they are all taken from camera drawings. The shading is diagram- 

 matic, and no attempt has been made to show cells, except in a rough 

 way in the younger stages of the auditory epithelium. In other 

 organs, such as the young brain, any marking 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 actually 

 impossible one. In Figs. 15 and 16 the columella is the only part to 

 which this description applies. 



Fig. 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 ad- 

 vanced at the dorsal edge than it is at the ventral. On 

 the right the section passes through the anterior part of 

 the auditory involution. 



* Tli'' f-ri'l'ilynipli anil contents '.I' the perilymphatic canals arc left, wliil.c 



