THE CRANIAL GANGLIA IN AMEIURUS 315 



the dorso-lateral placode may simulate very closely a lateral line 

 organ (fig. 5), but that it does not give rise directly to these organs 

 since this growing point is found in segments where later there 

 are no lateral line organs. 



Miss Piatt's work marks in a way the culmination of the work 

 begun by Balfour in 1876. A more thorough analysis of the cere- 

 bral nerves of the adult was needed before the full significance of 

 the dorso-lateral and epibranchial placodes could be determined. 

 While she seems to place the dorso-lateral and epibranchial plac- 

 odes in the same category, the determination that not all cells 

 proliferated from the neural crest and epidermis go to form gang- 

 lia and that the dorso-lateral placodes do not in some cases give 

 rise directly to sense organs, but that these appear later, mark 

 distinct advances in our knowledge of the origin of structures 

 derived from the epidermis. Much confusion has arisen appar- 

 ently from ignorance of these facts. 



Locating a specific ganglion of known composition in the adult 

 and then tracing it to its origin would have prevented errors that 

 have arisen unavoidably from pursuing the opposite course and 

 rinding a segmental ganglion and nerve for every area in which 

 cells are being proliferated from the neural crest and ectoderm. 

 As a general conclusion from her work, she thinks that the root of 

 a sensory nerve is no index to the segmental value of that nerve, 

 the position of the nerve root being in a great measure the expres- 

 sion of the coordinate relations which the central nervous system 

 serves. Miss Piatt emphasizes another point of the greatest 

 importance; it is in connection with the distinction between defin- 

 itive ganglion cells and nerve fibres on the one hand, and the 

 anlagen from which these come on the other. Just as not all 

 cells derived from the ectoderm go to form ganglia and nerves, so 

 not all cells grouped about ganglia and the growing points of 

 nerves are concerned directly in the nervous functions of the nerves 

 and ganglia. 



The appearance of Strong's paper in 1895, and more particularly 

 of Herrick's in 1899, mark as it appears to the writer, a turning 

 point in the study of the cerebral nerves. Evidently no homology 

 of the cerebral nerves can be established without an exact knowl- 



