510 EMBRYOLOGY. 



recognisable ; from the third month onward the upper and posterior 

 part of the auricle grows out more from the surface of the head ; 

 and it acquires greater firmness upon the differentiation of the 

 auricular cartilage, which had already begun at the end of the 

 second month. 



SUMMARY. 



1. The most essential part of the organ of hearing, the mem- 

 branous labyrinth, is developed at the side of the after-brain above 

 the first visceral cleft from a pit-like depression of the outer germ- 

 layer. 



2. By closure the auditory pit becomes the auditory vesicle ; it 

 sinks down and becomes imbedded in embryonic connective tissue, 

 from which the cranial capsule is subsequently developed. 



3. The auditory vesicle acquires the complicated form of the 

 membranous labyrinth by various evaginations of its wall, and 

 becomes differentiated into the utriculus, with the three semicircular 

 canals, into the sacculus with the canalis reunions and the cochlea, 

 as well as into the recessus vestibuli, by means of which sacculus 

 and utriculus remain permanently connected with each other. 



4. The auditory nerve and the auditory epithelium, which are 

 at first single, are likewise divided as soon as the vesicle is 

 differentiated into a number of regions into several nerve-branches 

 (nervus vestibuli, n. cochleae) and nerve-terminations (the cristae 

 acusticae of the three ampullae, a macula acustica for the utriculus 

 and another for the sacculus, and the organ of COETI). 



5. The embryonic connective tissue, in which are enclosed the 

 auditory vesicle and the products of its metamorphosis, is differen- 

 tiated into three parts : 



(a) Into a thin connective-tissue layer, which is closely applied 

 to the epithelial wall and together with it constitutes 

 the membranous labyrinth ; 



(6) Into a gelatinous tissue, which becomes liquefied during 

 embryonic life and furnishes the perilymphatic spacer 

 (in the cochlea the scala vestibuli and the scala tym- 

 pani) ; 



(c) Into a cartilaginous capsule, from which there arises by a 

 process of ossification the bony labyrinth. 



6. The middle and outer ear are derived from the upper part 

 of the first visceral cleft (the spiracle of Selachians) and its 

 periphery. 



