184 THE OLFACTORY OEGAN, 



Organs of Sense. 



The olfactory organ. The olfactory pit is the latest formed 

 of the three organs of special sense. It appears during a stage 

 intermediate between / and K, as a pair of slight thickenings of 

 the external epiblast, in the normal vertebrate position on the 

 under side of the fore-brain immediately in front of the mouth 

 (PL XIY. fig. 1 and 2, ol). 



The epiblast cells which form this thickening are very co- 

 lumnar, but present no special peculiarities. Each thickened 

 patch of skin soon becomes involuted as a shallow pit, which 

 remains in this condition till the close of the stage K. The 

 epithelium very early becomes raised into a series of folds 

 (Schneiderian folds). These are bilaterally symmetrical, and 

 diverge like the barbs of a feather from a median line 

 (PL XIV. fig. 14). The nasal pits at the close of stage K are 

 still sejDarated by a considerable interval from the walls of the 

 brain, and no rudiment of an olfactory lobe arises till a later 

 period ; but a description of the development of this as an in- 

 tegral part of the brain has already been given, p. 178. 



Eye. The eye does not present in its early development any 

 very special features of interest. The optic vesicles arise as 

 hollow outgrowths from the base of the fore-brain (PL xiv. fig. 

 3, op.v), from which they soon become partially constricted, and 

 form vesicles united to the base of the brain by comparatively 

 narrow hollow stalks, the rudiments of the optic nerves. The con- 

 striction to which the stalk or optic nerve is due takes place 

 from above and backwards, so that the optic nerves open into 

 the base of the front part of the thalamencephalon (PL XIV. 

 fig. 13a, oj:) n). After the establishment of the optic nerves, there 

 take place the formation of the lens and the pushing iu of the 

 anterior wall of the optic vesicle towards the posterior. 



The lens arises in the usual vertebrate fashion. The epiblast 

 in front of the optic vesicle becomes very much thickened, and 

 then involuted as a shallow pit, which eventually deepens and 

 narrows. The walls of the pit are soon constricted off as a 

 nearly spherical mass of cells enclosing a very small central 

 cavity, in some cases indeed so small as to be barely recog- 

 nisable (PL XIV. fig. 7, 1). The pushing in of the anterior wall 



