XOSE AND FA'E 



569 



the dorsal roots of the spinal nerves, while others arise as direct ventral 

 outgrowths from the brain, like the ventral roots. 



'I'he 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. 



/"iJ' 



t.st 



Fig. 149. — Early (A) and later (D) stages In the development of the eye. 

 (f it'll, diencephalon ; I'jrj. I. invagination of" ectoderm to form lens ; /. lens ; opt. c 

 oiiter layer of optic cnp ; opt. d . inner layer ; opt. St. optic stalk ; opt. v. optic 

 vesicle ',p^i. pharyn.v '.pty. pituitary body. (From Parker and Has\vell"s Zoolo^v 

 altered from Marshall.) 



the internal nostrils (in air-breathing 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. 149, A, opt. v) — is given off from each side 

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

 where it meets with an in-pushing of the ectoderm {inv. I) 

 which becomes thickened, and finally, separating from the 

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



