Chap. VII.] THE GENESIS OF TISSUES AND ORGANS. 129 



somewhat differentiated, thus giving rise to the so-called dental 

 capsule (d. c.). (1) The external layer of the papilla is com- 

 posed of columnar nucleated cells, the odontoblasts. These give 

 rise to long external processes, which lie in the tubules of the 

 dentine (d.), and by which that substance is formed. (2) The 

 internal layer of the enamel germ (enamel cells) sends processes 

 inwards which are believed to give rise to the enamel prisms of 

 the enamel (en.). (3) The internal cells of the dental capsule 

 surrounding the root of the tooth form an osteogenetic layer, 

 from which the cement is formed. 



Early in its development the enamel organ of the milk-tooth 

 gives rise to an outgrowth, into which a dental papilla soon 

 projects, and which is the enamel germ of the permanent tooth. 



The Organs of Smell, Sight, and Hearing (1.) Smell. 

 The organ of smell arises as two nasal pits lined with thick- 

 ened epiblast. As development proceeds and the olfactory 

 chambers are formed, this layer of epiblast spreading over their 

 surface, and over the surface of the turbinal bones or cartilages 

 developed within them, becomes of considerable extent. Very 

 early in development (in the chick on the third day) the olfac- 

 tory nerve-fibres, outgrowths from the anterior end of the brain, 

 come into relation with this olfactory membrane. 



(2.) Sight. Examine the fresh eye of an ox, sheep, or failing 

 these, rabbit. The anterior transparent cornea (Fig. 42, D., cw\) 

 is continuous with the opaque, fibrous sclerotic (scl.). These 

 form the capsule of the eye. . Make an incision in the cornea 

 with the point of the scalpel; a lymph-like transparent fluid 

 oozes out ; this is the aqueous humour contained in the anterior 

 chamber of the eye (aq. h.). Eemove the cornea and examine 

 the iris (z'r.), a circular curtain with a central perforation, the pupil 

 (pu.), through which the transparent crystalline lens (I.) is visible. 

 The iris contains circular and radiating muscular fibres, by the 

 contraction or relaxation of which the pupil can be enlarged or 

 diminished according as the light is dim or bright. The 

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