2o8 AVERS. [Vol. VI. 



About the fifth week the parts of the adult ear are all well 

 marked out. The two primary canals, anterior and posterior 

 verticals, inclose between them an angle of i6o° and open into 

 the utriculo-sacculus by a short common tube, while all three 

 present the S-shaped curvings so characteristic of the canals in 

 many fishes. The horizontal canal remains still a shallow groove 

 in the utricular wall. 



The nerve end organs of the utriculus lie very close together, 

 and at this time one cannot distinguish separate nerve branches. 

 The fibres of the utricular branch spring from the whole sur- 

 face of the utricular ganglion and pass in the form of a loose 

 bundle to the inner face of the utriculus, its sense organ having 

 divided. The fold has appeared in the cochlea, which later 

 development proves to be the beginning of the organ of Corti, 

 i.e. the forerunner and parent organ. 



About the second month the parts of the internal ear have 

 assumed their adult condition, with the exception of the epithe- 

 lial sensory end cells, which have not as yet been perfected, 

 though very distinctly modified from the remaining cells of the 

 ampullae and utriculus and sacculus. The canals and ampullae 

 are well formed, and each canal has a well-developed ama. The 

 ama of the external canal is the only one to persist in the adult. 

 The utriculus and sacculus are connected only by the split en- 

 dolymphatic duct, i.e. by the divided surface canal, and the 

 cochlea has nearly separated from the sacculus, retaining only the 

 slender canalis reuniens as a tubular communication. The cochlea 

 is at this time spirally twisted one and a half turns, the most 

 important change in the form of the canal since the last stage. 



The trunk of the acoustic nerve lies very closely applied to 

 the anterior wall of the auditory capsule (excepting the aquae- 

 ductus vestibuli). One cannot find a layer of mesoderm cells 

 between the ear capsule in Man. This antero-mesial portion of 

 the ear capsule contains the entire amount of the thickened 

 epithelial plate that later is to form the nerve end apparatus of 

 the adult internal ear, and consequently all these parts are held 

 to have direct relations to the auditory ganglion at this stage. 

 The posterior semicircular canal forms an exception to this, for 

 it receives its nerve supply at a relatively late period of life. 

 His was unable to find a nerve branch running to the posterior 

 ampulla earlier than the sixth week. This condition of things 



