THE EYE. 77 



is produced. The form becomes complicated by the development of the turbinal 

 folds (Fig. 212) and later by the addition of outgrowths from each nasal pit to 

 form the accessory sinuses of the adult nasal cavity. The tissue between the two 

 nasal pits forms the septum (Figs. 220 and 212, Sept}. The epithelium of each 

 pit sends a special invagination into the nasal septum to make the gland-like struc- 

 ture known as Jakobson's organ (Fig. 219, Jk.o). It will be seen from the above 

 that the extent of the nasal epithelium becomes very great, but only a small area 

 is concerned in the formation of the olfactory organ proper. This area underlies 

 the olfactory bulb of the fore-brain, and is termed the olfactory epithelium. The 

 cells of this become elongated and acquire boundaries from one another so as to 

 make a cylinder epithelium. Some of the cells become the^o-called olfactory cells, 

 which are more or less isolated and separated from one another by the intervening 

 supporting cells. The olfactory cells develop on their free ends a few small pro- 

 jecting hairs, and from their basal ends a single thread-like prolongation, which 

 becomes a fiber of the olfactory nerve. This fiber penetrates the olfactory bulb, 

 and there has its terminal arborization, which enters into special relations with the 

 mitral cells of the bulb. All the fibers of the olfactory nerve arise in this way; 

 hence the nerve so far as its development is concerned is unique in vertebrates. 

 It differs permanently from all other nerves in that its fibers never acquire any 

 medullary sheaths, because no cells migrate from the medullary wall or brain into 

 this nerve as they do into all others. 



Later, the complete separation of the nasal and oral cavities is accomplished 

 by the development of the palate shelves which grow out from the walls of the oral 

 cavity until they meet in the median line and unite with the lower edge of the 



nasal septum, as described and illustrated in Chapter VI. 







The Eye. 



The eye has a complicated history. The optic nerve and retina arise from the 

 medullary tube (brain). The lens arises from .the visual plakode. The remaining 

 structures of the eye are of mesodermal origin. 



Optic Vesicles. The optic vesicles arise very early from the extreme cephalic end 

 of the medullary tube, as two lateral outgrowths (Fig. 131), each of which soon ap- 

 pears quite as large as the central portion of the medullary tube which produces 

 it (see Fig. 133). The central portion of the tube, however, grows much more 

 rapidly than the optic vesicles in order to form the brain. The distal portion of 

 the optic vesicle expands, and we thus get the condition indicated in figure 154, Op. 

 The vessel may now be said to be stalked. The stalk is the anlage of the optic 

 nerve. The larger distal portion of the optic vesicle gives rise to the retina. The 

 optic vesicle comes in contact with the ectoderm, and over the area of contact the 

 visual plakode is differentiated. In the next stage the differentiation between the 

 eyeball and optic nerve is clearly seen (Fig. 154). This is accomplished by modifi- 

 cations in the distal portion of the optic vesicle. Its outer wall becomes invagi- 



