MARINE TKLEOSTEAX DEVELOPMENT. 83 



of the epiblast, considerably behind the latter organs. Then, in 

 the radial arrangement of cells, a cavity appears ; and this 

 at first, in most bony fishes, has the form of an elongated 

 slit, then it becomes broader and finally rounded. The walls 

 are originally very dense, but become thinner subsequently. 

 Otoliths occur about the sixth day, small at first, afterwards as 

 two, occasionally three, distinct rounded calcareous masses. 

 The otocysts change shape becoming like an oyster-shell. 

 Sensory cushions and semicircular canals are later developed, 

 so that in the post-larval fish the auditory apparatus becomes 

 complex. 



Olfactory pits and nerves. 



The olfactory pits are distinguishable on the sixth day or 

 thereabout, as paired thickenings of the sensory epiblast in 

 front of the upper part of the hemispheres. Each soon forms a 

 flat oval sac of slightly elongated cells. The nerves are minute 

 proliferations of the wall of the anterior fore-brain, which 

 coalesce with the proximal surface of the nasal pit. Each pit 

 has at first a single opening, but later a slight promontory 

 appears in the middle of each lip and in a few days a junction 

 occurs. The bridge becomes broad and each aperture is sur- 

 rounded by an elevated rim. 



Cranial nerves. 



The embryos are unfavourable for tracing the development 

 of the third, fourth and sixth, but the fifth (Trigeminal) is 

 a large nerve which springs from the upper lateral margin 

 of the hind-brain at a late embryonic stage. Just as it emerges, 

 it separates into several branches, the maxillo-palatine and man- 

 dibular, each having a large ganglion from which other twigs 

 pass. 



Behind the foregoing nerve, the seventh and eighth arise in 

 close proximity, the auditory being posterior and exhibiting 

 a large ganglion. Fibres from the former can be traced to 

 the base of the third ventricle or more correctly, above the 

 pyramids, whilst the eighth or auditory consists of thin fibres 



62 



