THE ORGANS OF THE OUTER GERM-LAYER. 



513 



The organs (rk) consist of numerous fine, rod-like cells, which at 

 their free ends bear fine bristles and are united into bundles that 

 are distinctly delimited from the ordinary cells of the epidermis. 

 They closely resemble the sensory nerve-terminations which are abim- 

 d;intly and widely distributed in the epidermis of Fishes and other 

 lower Vertebrates the beaker-like oryans or the nervous end-buds. 

 BLAUE has therefore named them olfactory buds. He proceeds from 

 the conception that, like the similarly constructed gustatory buds 

 of the oral cavity, they are descended from the sensory organs 

 distributed over the whole integument. The organ of smell is 

 simply a depressed patch of the skin richly provided with terminal 

 nerve-buds, which, undergoing a change of function, has come to sub- 

 serve a specific sense. The continuous 

 olfactory epithelium of the higher Ver- 

 tebrates has arisen from the originally 

 scattered, isolated olfactory buds (fig. 

 287 rk} by a process of fusion, the in- 

 different epithelium (fe) having gradu- 

 ally disappeared. In certain species of 

 Fishes and Amphibia such a transition 

 can be demonstrated. 



The further development of the organ 

 of smell is especially characterised by 

 the olfactory pits coming into relation 

 with the oral cavity. Each of them 

 (fig. 286) develops a furrow which 

 runs downward to the upper margin 

 of the mouth and receives on its outer 



side the previously described lachrymal groove, coming in an oblique 

 direction from the eye. Nasal pit and nasal furrow become deeper 

 in older embryos (fig. 288), owing to their margins protruding out- 

 ward as ridges and forming the so-called inner and outer nasal pro- 

 cesses. The two inner nasal processes are separated from each other 

 by a shallow furrow running from above downward ; they together 

 produce a thick partition between the two olfactory pits that in the 

 higher Vertebrates subsequently becomes more and more reduced in 

 thickness. They also furnish the middle of the roof <i)f the mouth. 

 The outer nasal processes (also called the lateral frontal processes by 

 His) form on either side a ridge protruding between the eye and the 

 organ of smell, and furnish the material for the formation of the 

 lateral walls of the nose and the alee. Their lower margins meet 



33 



Fig. 288. Fundament of the nose and 

 the roof of the primitive mouth- 

 cavity of a human embryo (C. II. 

 of His), seen from below after 

 removal of the lower jaw. From 

 His, "Menschliche Embryonen." 

 Magnified 12 diameters. 



