THE ORGANS OF SPECIAL SENSE. 579 



into the underlying mesoderm where they ramify to form the ducts and tubules. 

 The anlagen of the ducts and tubules of these glands are thus at first solid cords 

 of cells, their lumina being formed later by a breaking down of the central cells 

 of the cords. 



At the inner angle of the conjunctiva there develops beneath the eyelid 

 folds a third much smaller fold. This becomes the plica semilunaris which 

 in man is a rudimentary structure, but in many of the lower Vertebrates, 

 especially Birds, forms a distinct third eyelid, the so-called nictitating mem- 

 brane. A few hair follicles and sebaceous glands develop in a portion of this 

 fold forming the lacrymal caruncle. 



The Lacrymal Duct. At a certain stage in development, a groove bounded 

 by the maxillary process and the lateral nasal process extends from the eye to 

 the nose (Fig. 136). This is known as the naso-optic furrow. The ectoderm 

 (epithelium) lying along the bottom of this groove thickens about the sixth 

 week and forms a solid cord of cells. As development proceeds and the parts 

 close in, this cord of ectoderm becomes enclosed within the mesoderm, excepting 

 at its ends where it remains connected with the surface ectoderm of the eye and 

 nose, respectively. By a breaking down of the central cells of this cord a lumen 

 is formed and the cord becomes a tube, the lacrymal duct. The primary con- 

 nection of the lacrymal duct is with the upper lid, but while the lumen is being 

 formed an offshoot grows out to the under eyelid to form the inferior branch 

 of the lacrymal duct. 



THE NOSE. 



The anlage of the organ of smell is apparent in human embryos of about 

 three weeks as two thickenings of the ectoderm, one on each side of the naso- 

 frontal process. To these thickenings the term olfactory placodes has been 

 applied (Kupffer). A little later (in embryos of about four weeks), the placodes 

 become depressed below the surface, the depressions themselves being the 

 nasal pits or fossa (see p. 148; also Fig. 123). The placodes, which are 

 destined to give rise to the sensory epithelium, thus come into closer relation 

 with the olfactory lobes of the brain (rhinencephalon) which represent out- 

 growths of the fore-brain (telencephalon) (see p. 501). 



As described in connection with the development of the face, the lateral 

 nasal process arises on the lateral side, the medial nasal process on the medial 

 side, of each nasal pit (p. 148 et seq.; also Fig. 134). Of these processes, the 

 lateral is destined to give rise to the lateral nasal wall and the wing of the nose, 

 the medial to a part of the nasal septum (see p. 148). As development pro- 

 ceeds, the epithelium (ectoderm) of the nasal fossae grows still deeper into the 

 subjacent mesoderm, the fossse thus becoming converted into the nasal sacs, 

 which lie above the oral cavity. According to Hochstetter and Peter, the 



