NOSE AND EYE 



587 



The olfactory organs arise as sac-like invaginations of the 

 ectoderm, one on either side of the snout, and become 

 enclosed by the cartilaginous olfactory capsules, developed, 

 with the rest of the skeleton, from the mesoderm. The 

 aperture of invagination gives rise to the external nostril, 



inv I m ': die 



FIG. 160. Early (A) and later (B) stages in the development of the eye. 

 dicn. diencephalon ; inv. 1. invagination of ectoderm to form lens ; /. lens ; opt. c. 

 outer, and opt. d . inner layer of optic cup ; opt. st. optic stalk ; opt. v. optic 

 vesicle \ph. pharynx \pty. pituitary body. (From Parker and Haswell's Zoology, 

 altered from Marshall). 



the internal nostrils (in pulmonate forms) being developed 

 subsequently. 



The mode of development of the paired eye of Vertebrates 

 is peculiar and characteristic. 



At an early stage of development a hollow outgrowth the 

 optic vesicle (Fig. 160, A, opt. v) is given off from either side 

 of the fore-brain and extends towards the side of the head, 

 where it meets with an in-pushing of the ectoderm (inv. 1} 

 which becomes thickened, and finally, separating from the 

 ectoderm, forms a closed, spherical sac (B, /) with a very 

 small cavity and thick walls (compare Fig. 64, i, e). This 

 body is the rudiment of the lens : as it enlarges it pushes 



