[53] EMBRYOGRAPHY OF OSSEOUS FISHES. 507 



origin would seem to be carried downwards like that of the optic nerves 

 or stalks as they may at first be called ; but of course the olfactory 

 nerve develops later than the optic. In relatively late stages, or after 

 hatching, transverse sections show the roots of the olfactory nerves 

 arising from the sides of the cerebral lobes and passing to the nasal pits. 

 As the snout develops, however, the nasal organs of the young fish as- 

 sume a more elevated and posterior position, while in some cases, in 

 consequence of the great forward development of the vomer, parasphen- 

 oid, ethmoid, premaxillary, prefrontal, and maxillary bones, the nasal 

 organs are carried very far forwards so that the olfactory nerve may 

 require to be prolonged several inches in the adult before it reaches the 

 nasal organ. 



The auditory organs, like the nasal, are involutions of the sensory 

 layer of the epiblast. In the cod their rudiments begin to develop on 

 about the tenth day; by the twelfth, at first apparently solid, their in- 

 volution has been completed, and they are present on the fourteenth as 

 a pair of very thick- walled ovoidal vesicles, with a very small cavity, as 

 shown in Fig. 26, au. This internal cavity gradually increases in size, 

 while the walls themselves become gradually thinner. The contents of 

 the vesicle is apparently a fluid lymph; on the fifteenth day two very 

 refringent bodies make their appearance in the auditory vesicle on its 

 internal wall ; these are the otoliths, the asterisJc and sagitta, as they are 

 named, respectively. With the development of the auditory vesicle 

 they increase somewhat in size; they are calcareous in composition and 

 have a depressed spheroidal form, with a radiate fibrous structure. 

 The exact mode of development of the otoliths is not well known. With 

 the progress of development the auditory vesicles elongate somewhat 

 antero posteriorly, the inner portion becomes vestibular in character, 

 and the otoliths lie against its inner wall, near the ventral border 

 of the sack. At the time of its involution it would appear that the au- 

 ditory nerve or its rudiment was developed from the side of the hind 

 brain. Sections of the later stages through the auditory vesicles show 

 the roots of the auditorj'^ nerves arising from the side of the medulla, 

 pretty high up, and curving down on the inner face to the lower anterior 

 part of the iuner side of the auditory vesicle. They enter the vesicle 

 on its lower inner side, and terminate in a cushion of columnar epithelial 

 cells, which are surmounted by fine, hair-like protoplasmic filaments, 

 which project freely into the endolymph of the utriculus, as we may 

 call that portion of the auditory vesicle at this stage of its development. 

 The sensory terminal cushion, in which the auditory nerve ends, is evi- 

 dently an acoustic macula; in its vicinity pigment cells are usually 

 developed in considerable numbers some time after incubatiou. The 

 horizontal anterior and posterior semicircular canals are apparently 

 developed by the infolding of the walls of the auditory vesicle. These 

 folds first appear as ridges, which apparently grow inwards in such a 

 way as to shut off the semicircular canals from the vesicle, except at 



