338 



AYERS. 



nerve of an adult Raja erinacea, X S- The branching of the utricular nerve indica- 

 tive of division of the original sensory patch is well shown. Osmic acid preparation. 



Fig. II. The two rami of the auditory nerve of Myxine glutinosa and their 

 branches, after Retzius. X i8. The rami anterior and posterior supply respectively 

 the utricular and saccular chambers of the ear, neither one crossing the line of sepa- 

 ration marked out by the endolymphatic duct. 



Fig. 12. Horizontal section through the auditory capsule of Lacerta embryo, 

 after Hoffmann. X 50. The position of the vesicle is seen to be between the 

 VH-VIII and the IX nerves, which come into immediate contact with the walls of 

 the vesicle at this stage of development. 



Fig. 13. Horizontal section through the ear of Petro77iyzon marhius, to illustrate 

 the relation of the ramus accessorius of the facial nerve to the ventral walls of the 

 ear. In sections dorsad of the one figured, the nerve rises nearly vertically in the 

 sulcus frontalis, and gives off a branch to the sense organ in the commissural chamber, 

 but its main portion is continued outward to the stirface canal organ, which it supplies. 

 For a considerable part of its course the nerve runs parallel and more or less closely 

 applied to the ramus anterior of the auditory proper. X 70. c, the outer walls of 

 the connective tissue sheath of the ear. r.ac. VIH, the accessory branch of the 

 facial. 



Fig. 14. A transverse section through the ear of a young Petromyzo7i Planeri, 

 near the middle of its vertical diameter. The section passes through the two endo- 

 lymphatic ducts, the canals and ampuUse where the latter open into the utricular and 

 saccular compartments. X 25. cr.ac, the sense organs of the anterior and poste- 

 rior ampulte. c.c, the cartilaginous capsule. 



Fig. 15. A surface view of the superficial sense organs of the lateral line system 

 of the Dogfish (^Galeus cants), illustrating their relation to the auditory sense organ 

 canal or endolymphatic duct. The auditory organ is subject to a precocious develop- 

 ment, but retains its relation to the surface of the body and to its superficial relatives 

 for a long time after disappearing from the surface; and, as shown in the figure, 

 retains its place in the line of sense organs, connecting the lateral systems over 

 the aural region. X 20. o.d.e., the obliquely placed mouth of the endolymphatic 

 duct; au.g., the groove of modified epidermal epithelium (ectoderm), which reaches 

 to a.s.o., the point at which the auditory organ first made its appearance on the 

 surface of the body. 



Fig. 16. A dissection of the surface and ear canals of an Elasmobranch (^Rajoid) 

 embryo, probably Torpedo. X 10. The ears are revolved somewhat inwards and 

 backwards on their endolymphatic ducts, in order to show their canals to advantage, 

 but otherwise the parts are in their relative positions as viewed from the dorsum. 

 The ears communicate by means of their endolymphatic ducts with the large sub- 

 dermal sac, and these by means of the tubes d.e., ordinarily described as the con- 

 tinuation of the endolymphatic ducts, with the exterior opening on the surface of the 

 head at the points p.ac, or the pori acustici, from which points grooves are contin- 

 ued to t in the transverse line of sense organs, out of which the auditory organ 

 migrated at the beginning of its involution, s.e., endolymphatic sacs; o.c, occipita 

 canal or line of sense organs; c.s.o., canal sense organ. The pink elevations s.s. in 

 endolymphatic sacs indicate the position of probable sense organs. 



Fig. 17. The crista abortiva of an adult Raja ocellata showing a supernumerary 

 bipartite sense organ with distinct nerve branch. X 3. Osmic preparation. 



Fig. 18. A surface view of the external openings of the endolymphatic duct of 

 Raja ocellata. X 2. 



