Johnston, Morphology of the Head. 217 



The origin of the nerves from ectodermal placodes has led to the 

 development of the hypothesis of primary and secondary sense 

 organs, to which the Hertwigs, Retzius, v. Kupffer and 

 others have contributed. The hypothesis may be stated as fol- 

 lows. The dorsal-lateral placodes in the vertebrate embryo 

 represent a series of primary sense organs in which the sensory 

 cells were of a type illustrated by the cells of the olfactory or- 

 gan in vertebrates or by the usual sense cells of invertebrates. 

 Such sense cells send their own axones into the central nervous 

 system. There has been a gradual sinking of these sense cells 

 beneath the surface comparable to the migrating inward of sen- 

 sory epithelial cells in invertebrates. The sinking in of cells 

 from the primary sense organs is repeated in the ontogeny and 

 the cells become the ganglion cells of the lateral line nerves. 

 Then there appear in the placodes newly differentiated sense 

 cells, the neuromast or pear cells characteristic of lateral line 

 organs which constitute secondary sense organs. 



It should be noticed with regard to this account of the 

 lateral line organs and nerves, first that there is no direct evi- 

 dence for this interpretation of the placode cells; and second 

 that it does not imply any close connection between this system 

 and the general cutaneous system. Also, it seems to the writer 

 that a somewhat more simple explanation of the origin and 

 relationship of the neuromast system may be found. The above 

 hypothesis postulates the existence primarily of the sense organs 

 of the invertebrate type, the transformation of these into ganglia, 

 and the new formation of sense organs of the neuromast type. 

 While the transformation of sense cells into ganglion cells is not 

 unthinkable, yet the transformation of a cell possessing a 

 specialized sensory peripheral process into one whose peripheral 

 process is adapted to receive stimuli from another cell with 

 which it enters into complex relations is a very profound change. 

 Moreover, what advantage is gained by the replacing of primary 

 sense cells by secondary sense cells and the addition of one link 

 to the sensory chain is difficult to see. Any change which in- 

 creases the length of, and the number of relay stations in, a re- 

 flex arc would generally be considered to be disadvantageous. 



