No. I.] THE VERTEBRATE EAR. 309 



9. The Cyclostomes and some Elasmobranchs possess two 

 endolymphatic ducts, or surface canals, on each side c^f the 

 head, one of which on either side bears a canal sense organ 

 supplied, so far as our anatomical knowledge reaches, by the 

 facial nerve, or more exactly by the so-called auditory accessory 

 nerve, which is said to be a branch of the facial. In this 

 connection Mayser's conclusions as to the relation of all 

 canal organ nerves to the auditory proper should be borne in 

 mind. 



10. The otoliths in the ear chambers are to be considered as 

 essentially foreign bodies which are tolerated because of the 

 impossibility of getting rid of them. They are the result of 

 the secretive action of the ectoderm cells, which in ancestral 

 forms produced the surface scales. The higher we rise in the 

 vertebrate series, and also the greater the auditory sensitiveness, 

 the fewer the otoliths found in the ear. 



Those sense organs whose condition indicates their predomi- 

 nance in auditory work have no otoliths. 



11. It is found that the sense organs of the cochlea undergo 

 a series of transformations heretofore overlooked and that there 

 is an organ in the adult cochlea of the hydrosaurian Reptilia 

 which makes its appearance in the embryonic mammal, but 

 together with its nerve supply fades away before adult life is 

 reached. Whereas the cochlear organ of birds, for all that we 

 know to the contrary at present, seems to be of the form of an 

 undifferentiated plate of sensory cells, in the Alligator and in 

 mammals the organ of Corti becomes differentiated into a linear 

 series of organs or cell groups, which are individually morpho- 

 logically equivalent to the other simple sense organs of the ear, 

 e.g. the cristae acusticae. This segmentation of the organ of 

 Corti is indicated by {a) the hair cells, {b) the supporting cells, 

 if) the blood-vessels, {d) the nerves, and consequently the skele- 

 ton (the lamina spiralis ossea) and the soft parts (the membrana 

 basilaris) of the floor, transmitting these last two structures, 

 c and d. 



12. The cells which line the sulcus spiralis internus in the 

 adult are all that are left of the large epithelial ridge of the 

 embryo. This ridge is composed of long columnar cells which 

 grow shorter and disappear. It is not yet ascertained what the 

 mode of disappearance is. It may be that the columnar cells 



