THE EYE AND EAK. 541 



unite with each other, thus closing the eye, about the third or 

 fourth month, and separate again shortly before birth. 



The lacrymal duct is formed along the line of the lacrymal 

 groove, as a linear depression running from the eye to the nose, 

 along the line of meeting of the external nasal process and the 

 maxillary arch (Fig. 207). The duct itself arises as a solid rod 

 of epithelial cells split off from the floor of the groove : it 

 becomes sinuous at an early stage, and from its sides the lacry- 

 mal glands arise as solid branched outgrowths, with dilated, 

 hollow, bulb-like ends. At a later stage the solid cords become 

 hollow along their axes, and converted into the lacrymal 

 ducts. At the inner canthus of each eye the duct bifurcates, 

 while still a solid rod, to form the rudiments of the upper and 

 lower lacrymal canals. 



The third eyelid, or plica semilunaris, which is rudimentary 

 in man, arises as a small fold of the conjunctiva at the inner 

 canthus of the eye, within the upper and lower eyelids. 



3. The Ear. 



The ears appear as a pair of open pits at the sides of the 

 hind-brain on the fifteenth day (Fig. 197, El). Almost directly 

 afterwards the mouths of the pits close, and the vesicles, thus 

 formed, separate from the skin. The original mouth of each pit 

 lengthens out into an elongated neck, the recessus labyrinthi 

 (Fig. 229, ER), while the vesicle itself forms a flattened sac, EV, 

 somewhat oval in outline, and lying embedded in the connective 

 tissue at the side of the hind-brain. 



At the commencement of the fifth week the auditory vesicle 

 becomes more irregular in shape. Its ventral and anterior end 

 (Fig. 227, EI) grows forwards as a short blunt process, which 

 forms the rudiment of the cochlea ; while, near its dorsal end, 

 three flattened projections appear on its outer surface, which 

 are the first stages in the formation of the three semicircular 

 canals. 



By the end of the fifth week (Fig. 230), the auditory vesicle 

 has increased considerably in size, and its main divisions are 

 well established. The body of the vesicle is divided by a fold 

 into two main portions : a dorsal division, or utriculus, UT ; and 

 a ventral division, or sacculus, s. From the utriculus the three 

 semicircular canals arise, the two vertical canals, EA, EP, being 



