136 THE FROG-. 



the head, lying at the sides of the fore-brain, and in front of, and 

 slightly dorsal to, the position in which the mouth will after- 

 wards appear. 



The two layers of epiblast soon lose their distinctness in these 

 patches ; and a pitting in of the surface, involving both layers, 

 appears in each of the patches. The pits so formed become the 

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

 anterior nares, and the epiblastic lining of the pits becoming 

 converted into the olfactory epithelium. The condition of these 

 pits at the time of hatching of the tadpole is shown from the 

 surface in Fig. 72, oc, and in horizontal section in Fig. 74, of. 



The olfactory pits rapidly deepen (Fig. 66, of), rather by 

 the upgrowth of folds of skin round their margins than by 

 depression of the floors of the pits themselves ; the result of 

 this process being the formation of a pair of deep pits, of which 

 the inner walls are derived from the original patches of olfactory 

 epithelium. 



A short time after hatching of the tadpole, a solid rod of 

 epithelial cells is formed by proliferation of the cells of the 

 floor of each olfactory pit. These rods of cells grow downwards 

 and inwards towards the roof of the pharynx, meeting and fusing 

 with this immediately behind the septum between the pharynx 

 and the stomatodaaum (Fig. 64). Shortly after the mouth open- 

 ing is established, by perforation of this septum, these rods of 

 cells become tubular ; and in tadpoles of 12 mm. length, in which 

 the hind limbs are just appearing, the tubes open into the roof 

 of the mouth as the posterior nares (Fig. 76, zi). 



By further folding of the walls, and by the formation of 

 caecal outgrowths from each sac, the complicated olfactory laby- 

 rinth of the adult is developed. A special diverticulum of the 

 ventral wall of each sac gives rise to the organ of Jacobson. 



2. The Eye. 



The eye differs from the other sense organs inasmuch as an 

 accessory part, the lens, is alone formed from the surface 

 epiblast ; while the sensitive part of the eye, or retina, arises as 

 an outgrowth from the brain, and thus is only indirectly derived 

 from the epidermis. 



The optic vesicles have already (p. 125) been described as a 

 pair of hollow outgrowths, which arise from the fore-brain about 



