THE OLFACTORY ORGAN. 407 



sack. In some other Vertebrates this nerve seems hardly to be 

 developed, but it is easily intelligible, that if in the ordinary 

 course of growth the olfactory sack became approximated to the 

 olfactory lobe, the nerve which grew out from the latter to the 

 sack might become so short as to escape detection. 



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 

 (PI. 15, figs, i 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 stare 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 (PI. 15, 

 fig. 14). The nasal pits at the close of stage K are still separated 

 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 integral part of the 

 brain has already been given, p. 401. 



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 (PI. 15, 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 (PI. 15, fig. 

 130, op.n}. After the establishment of the optic nerves, there 

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

 anterior wall of the optic vesicle towards the posterior. 



