THE NOSE AND EYE. 275 



surface of the fore part of the head ; these soon become depressed, 

 forming pits (Fig. 113, of), with the bottoms of which the 

 olfactory nerves very early become connected (p. 266). 



The mouths of the olfactory pits narrow, and become slit- 

 like, but remain open throughout life as the external nostrils 

 (Figs. 125, 126, ok). 



The epithelial lining of each olfactory pit becomes thrown 

 into folds, to increase its surface ; and gives rise directly to the 

 olfactory epithelium, or Schneiderian membrane, of the adult 

 nose. 



The posterior narial passage is a secondary formation ; it 

 appears at first as a groove on the under surface of the head, 

 leading from the edge of the olfactory pit to the anterior and 

 outer angle of the stomatodasum. This groove is well marked 

 on the fourth day, its inner lip being formed by the fronto-nasal 

 process (Fig. 125, fp), or median part of the face, between the two 

 olfactory pits ; and its outer lip being formed by the maxillary 

 arch (Fig. 125, Mx), or rudiment of the upper jaw. 



On the fifth day (Fig. 125) the olfactory groove deepens, 

 and its inner and outer lips, formed by the fronto-nasal process 

 and maxillary arch respectively, meet and coalesce, so as to 

 convert the groove into a tube, leading from the olfactory pit 

 to the mouth. This tube is the posterior narial passage ; it at 

 first opens into the anterior end of the mouth cavity, immediately 

 behind the upper lip ; but as the mouth elongates, by growth 

 forwards of the beak, a horizontal shelf-like partition is formed 

 at the anterior end of the upper jaw on either side. By fusion 

 in the median plane, the two horizontal partitions form the 

 palatal septum, which stretches across the anterior part of the 

 mouth, separating the olfactory or nasal region above from the 

 buccal cavity below, and shifting backwards the communication 

 between the posterior nostrils and the mouth. 



2. The Eye. 



As in the frog, and in Vertebrates generally, the retina or 

 essential part of the eye is formed from the optic vesicle, while 

 the lens is an independent invagination of the surface epiblast. 



The optic vesicles arise, at the commencement of the second 

 day, as a pair of hollow lateral outgrowths from the fore-brain ; 

 they grow rapidly, and attain some size before the lips of the 



T 2 



