122 DEVELOPMENT OF THE PROG 



the anterior end of the head, in the angles between the fore- 

 brain and the optic vesicles. A pitting-in of the surface, 

 involving both layers of the epiblast, soon appears in each of 

 these thickenings, and the pits so formed become the nasal 

 sacs ; the mouths of the pits forming the nostrils or anterior 

 nares, and the epiblast lining the pits giving rise to the olfac- 

 tory epithelium. 



Erom the inner or deeper end of each olfactory pit a diverti- 

 culum, at first solid, but soon becoming hollow, gi-ows down- 

 wards to the roof of the pharynx, into which it opens,^a.s the 

 posterior nares, very shortly after the formation of the mouth 

 opening. 



Fig. 30. — Half sections in the transverse plane of a tadpole 10 mm. 

 long (left half) and of a tadpole 12 mm. long (right half). X 35. 



BF, fore-brain ; OD, outer wall of optic cup (pigment layer of adult 

 retina) ; OC, inner wall of optic cup (remainder of adult retina) ; 

 OL, lens, attached to epiblast in younger tadpole, but forming a 

 hollow vesicle at the later stage ; TP, pharynx ; Q, sucker. [G. H. F.] 



The Eye. The eye differs from the other sense organs, inas- 

 much as the lens alone is formed directly from the surface epi- 

 blast, while the sensitive part of the eye, or retina, arises as an 

 outgrowth from, the brain. The optic vesicles have already been 

 described as arising at a very early period as lateral outgrowths 

 from the fore-brain ; these soon become constricted at their 

 necks so as to be connected with the brain by narrow stalks, 

 which ultimately become the optic nerves. 



The outer surface of each optic vesicle, which is at first in 

 close contact with the surface epiblast, soon becomes flattened 

 (Fig. 30, left half), and then thickens so greatly as almo.st to 



